Although blue foods exist, they're rare enough compared to other foods for food preparers to use blue plasters as a convention. The natural colour of a given food is due to pigments that have some biological origin. Is there any evolutionary reason why these are rarely blue?
Blue color is not only rare in edible organisms - Blue color is rare in both the animal and plant Kingdoms in general. In animals, blue coloring is generated through structural optic light effects, and not through colored pigments. In the few blue-colored plants, the blue color is generated by blue pigment, namely anthocyanins. The reason for the scarcity of blue pigments remains unknown as far as I know.
The vast majority of animals are incapable of making blue pigments, but the reason appears to be unknown, according to NPR. In fact, not one vertebrate is known to be able to. Even brilliantly blue peacock feathers or a blue eye, for example, don't contain blue pigment. Instead, they all rely on structural colors to appear blue. Structural colors are brought about by the physical properties of delicately arranged micro- and nanostructures.
Blue morpho butterflies are a great example of a brilliant blue color brought about by structural colors. Morphos have a 6-inch wingspan — one side a dull brown and the other a vibrant, reflective blue. The butterflies have tiny transparent structures on the surface of their wings that scatter light in just the right way to make them appear a vibrant blue. But if you grind up the wings, the dust — robbed of its reflective prism structures — would just look gray or brown.
Similarly, in the Kingdom of plants less than 10 percent of the 280,000 species of flowering plants produce blue flowers. In fact, there is no true blue pigment in plants and blue is even more rare in foliage than it is in flowers. Blue hues in plants are also generated by floral trickery with the common red anthocyanin pigments. Plants tweak, or modify, the red anthocyanin pigments to make blue flowers, including pH shifts and mixing of pigments, molecules and ions. These complicated alterations, combined with reflected light through the pigments, create the blue hue (source: Mother Nature Network).
Although @AliceD's answer is a great simple demonstration of the rarity of blue in our natural world, there's likely a more nuanced/technical reason.
Blue light was the most available wavelength of light for early plants growing underwater, which likely led to the initial development/evolution of chlorophyll-mediated photosytems still seen in modern plants. Blue light is the most available, most high-energy light that continues to reach plants, and therefore plants have no reason not to continue taking advantage of this abundant high energy light for photosynthesis.
Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light. (Absorption of red light likely evolved once plants moved on land due to its higher efficiency).
Pigments appear as whatever color is not absorbed (i.e, they appear as whichever wavelength(s) of light they reflect). Because blue pigment would reflect the majority of light that modern plants rely on for their chlorophyll-mediated photosystems, blue pigments remain scarce in plants.
- Photosynthetic organisms would not remain competitive if they failed to absorb the readily available, high-energy blue light, and so therefore evolution has likely very infrequently favored the generation (or propogaiton) of blue pigments.
As this page from Humboldt State University demonstrates, blue and green light pass through the atmosphere to the earth's surface better than almost all other wavelengths of light:
transmission is when electromagnetic energy is able to pass through the atmosphere and reach the surface. Visible light, largely passes (is transmitted) through the atmosphere.
This means that blue and green light are the most available wavelengths of light.
Note that blue/green are followed closely by the rest of the visible spectrum and NIR (near infrared).
Also note that a large portion of ultraviolet is largely absorbed by atmospheric gases (primarily ozone), and therefore poorly transmitted.
It's important to understand that (from U. Wisconsin):
More energetic waves have shorter wavelengths while less energetic waves have longer wavelengths.
As a result, blue light (being the most available highest energy wavelength of light), seems to be the optimal wavelength of light for photosynthesis.
More info about the physics behind light energy can be found here.
Photosynthesizing organisms contain pigments (typically heme/porphyrin-based chlorophylls and various carotenoids) that absorb light energy. Basically, the energy from a photon of light raises an absorbant pigment to a higher energy state (called an excited state), and then the pigment releases that unstable energy to return to it's ground state -- this excess energy is what powers the biochemical reactions of photosynthesis. See here for more details.
As you can see, plants have evolved to have pigments that absorb primarily blue light (followed by red light). These pigments reflect green light and so therefore appear green.
- Presence of blue pigments (i.e., those that reflect highly abundant blue light), on the other hand, would be in direct contrast to these photosynthetic efforts. As a result, blue pigments remain rare in photosynthetic organisms powered primarily by chlorophyll-driven photosystems.
However a number of sources (e.g., Mae et al. 2000, Brins et al. 2000, and here) suggest that although plants absorb more blue light than other wavelengths, the most efficient photosynthesis does not occur using blue light. Instead, red light results in the highest photosynthetic efficiency.
One of the reasons that they (in this case Brins et al) found was that xanthophylls were dissipating the excess energy associated with blue light causing a rate decrease in blue-light photosynthesis.
This NIH page suggests that high-energy light isn't even necessary for plants:
Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield — the amount of photosynthesis per absorbed photon — is the same for all wavelengths of visible light shorter than 680 nm.
However, all of this may be for nought, because there is ample sunlight available to plants. Again from the NIH page:
However, even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sun, ≈1.2 × 1020 photons/m2/s), each reaction-center chlorophyll a absorbs about one photon per second, which is not enough to support photosynthesis sufficient for the needs of the plant. To increase the efficiency of photosynthesis, especially at more typical light intensities, organisms utilize additional light-absorbing pigments.
In other words, plants are not entirely photosynthetically efficient and do not typically use all of the light available to them. From Wikipedia:
Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case (see Photosynthesis-irradiance curve). Above about 10,000 lux or ~100 watts/square meter the rate no longer increases. Thus, most plants can only utilize ~10% of full mid-day sunlight intensity.
So in summary:
- Blue and green light are the most available wavelengths of light.
- Blue light is the most energy-rich of the high-availability wavelengths of light
- Plant pigments absorb mostly blue light
- BUT plants don't necessarily need the high energy of blue light for efficient photosynthesis.
So what gives?...
So given all of that, the question still remains: why absorb mostly blue light and not green light?
The answer, though still somewhat conjectural, is likely due to the light availability of early plants. Early plants evolved, like all life, under water.
It turns out, just like the variability in transmittance of different wavelengths of light through the atmosphere, certain wavelengths of light are more capable of penetrating deeper depths of water. Blue light typically travels to deeper depths than all other visible wavelengths of light. Therefore, the earliest plants would have evolved to concentrate on absorbing this part of the EM spectrum.
However, you'll notice that green light penetrates relatively deeply as well. The current understanding is that the earliest photosynthetic organisms were aquatic archaea, and (based on modern examples of these ancient organisms) these archaea used bacteriorhopsin to absorb most of the green light.
Early plants grew below these purple bacteriorhopsin-producing prokaryotes and had to use whatever light they could get. As a result, the chlorophyll system developed in plants to use the light available to them. In other words, based on the deeper penetrative ability of blue/green light and the loss of the availability of green light to pelagic prokaryotes above, plants evolved a photosystem to absorb primarily in the blue spectrum because that was the light most available to them.
So why have plants not evolved to use green light after moving/evolving on land? As stated above, plants are terribly inefficient and can't use all of the light available to them. As a result, there is likely no competitive advantage to evolve a drastically different photosystem (i.e., involving green-absorbing pigments). So earth's plants continue to absorb blue light and reflect the green, and blue pigmentation remain uncommon in our world.
- Plants likely evolved to produce anthocyanins (the pigments responsible for the reds/blues/purples attributed to blueberries and violets) for reasons other than photosynthesis -- e.g., attraction, UV protection, or even protection against herbivores. See here, here, and here for examples.
So what about non-plant organisms?
According to Wikipedia:
- Carotenoids are the most common group of pigments found in nature, including in animals.
- Carotenoproteins complexes are responsible for the various colors (red, purple, blue, green, etc.)
- Animals are incapable of making their own carotenoids and thus rely on plants for these pigments.
In other words, most pigments in non-plant organisms originate either directly or biochemically from the organism's diet. Without direct ingestion of blue pigments, these chemicals are unavailable or biochemically expensive to produce (see Crustacyanin). As a result, blue pigments are also uncommon in animals.
Though, as AliceD's sources point out, we still don't fully understand why animals don't produce more blue pigments.
It is not that there are no blue foods, it is that the English language does not like calling foods "blue".
There are no natural borders between "colors" in a colorspace, all colors we name (and learn to distinguish) are culturally defined. So, an important thing to recognize is that, a color somebody calls "blue" can be called "purple", "red" or "maroon" or "green" or something else.
With that in mind, we can take a look at foods. Animal foods on the inside are protein, and cooked protein is usually a pale greyish goo (although muscle cooked medium still has reddish coloring). One of the rare examples would be some eggs with blue shells. In plants, basically all leaves we eat are colored by chlorophyl. So they are out of the equation.
Other plant parts, especially fruits, tend to be colored by one of a few other groups of pigments. And a major group of those pigments, the anthocyanins, ranges from blue to purple to red (they also change with pH). So a ton of foods have a somewhat purplish blue color, and are also named "blue" in other languages. Plums, aubergines, red grapes, red cabbage, and many berries are good examples for that. Then you have some plants which come in many colors, but the major cultivars happen to not be blue - potatoes and maize are the first to come to mind.
Other shades of blue are less common just because they don't happen to be covered by the major pigment groups, but if you go down into shade level, then "royal blue" is just one of many on the spectrum which is missing. Also note that there are still plants which have other shades of blue and are consumed, they are just not commonly thought of as "food". Lavender, chicoree, gentian and pansies all have blue flowers of various shades and are traditionally used in recipes.
So there is no special evolutionary reason why plants or animals should not be blue - many of them are blue, and we eat some of the blue ones. It is simply a cultural or linguistic trend for English speaking people to not see their food as blue.
I would argue that the more appropriate question to ask is: Why can we easily identify blue plasters, even though so few foods are blue? (And the main part of this answer is why this even is a reasonable question.) And the main answer to that is probably that a sufficiently large food-sparse region in colour space eventually got its own colour term, namely blue.
A colour is a region in colour space that speakers of the respective language agree to assign the same name. Basic colours are colours that you already know in kindergarten and wouldn’t consider a special case of another colour (like aquamarine being a special case of blue).
The evolution of colours
In their evolution, languages refine their segmentation of colour space step by step, usually starting with light-warm vs. dark-cool, then separating red, yellow, and green (further reading). The fifth segmentation usually splits blue and green – or more precisely: something similar. After this, English has white, black, red, yellow, green, and blue.
Now, languages split an existing colour when there is a relevant distinction to make within it. And potential food was the predominant thing for humans to distinguish for most of their history. If some fruit changing its colour from green to blue indicates that it has become edible, this is not only something relevant to communicate, but having a word for it makes it easier to remember this. By contrast, humans looked at the sky on a daily basis but whether it was green or blue was nothing they ever needed to communicate.
Consequentially, the colours of foods were a driving factor for how we segment our colour space to begin with. And here, the distinction between blue/green and purple/red seems to be dominated by exactly this: Most food that leans on blue just remains within what we call purple (e.g., plums and aubergines) and often, turning from green to purple indicates ripeness. (And in the case of blueberries, going from purple to blue indicates ripeness.) However, I am not aware of any food that turns from green directly to blue. So, there is some reason to assume that our colour space is segmented such that there is little food on the blue side of the blue–purple divide.
Feedback from language to perception
Another weird aspect of this is that language influences our perception: If we learn a word for a colour from young age, we also become better at distinguishing that colour. A striking example of this are the Himba people, whose language segments the colour space differently than English (and many other languages). They find it much more difficult to distinguish certain a certain pair of shades of blue and green (because their language doesn’t) but find it easier to distinguish certain shades of green (because they are different basic colours in their language)¹²³.
So, if the English language didn’t make the blue–green distinction, you likely wouldn’t have asked this question – not only because you would lack the words, but also because you wouldn’t even notice³ that there is a family of shades of green that you find in all sorts of technological products (and the sky), but not in food.
Now, what’s so special about blue?
There are several other colours you will rarely find in food, e.g., fuchsia/magenta, neon green, and brick red. Why did you not ask the question about them, and why don’t we use these colours for plasters for food preparers? The answer is probably that blue is a basic colour term but that is only half the truth: Because blue is a basic colour term, we are better at distinguishing it than e. g. neon green and thus it is more effective as a plaster colour.
This leaves the question:
Why did blue become a basic colour term and continued to exclude food?
The answer to this is naturally hidden within the mists of history. However, the appearance of some sort of blue–green distinction is a common trait of languages, suggesting that it is more than just chance. My educated guess it that what we call blue is simply the largest region of colour space that is sparse with food, for the reasons addressed in the answers by AliceD and Theforestecologist. Mind that a difficulty with this explanation is that it is somewhat subjective how we even measure colour-space volume (and thus define largest) in a way that reflects human perception.
In contrast to other basic colour terms, blue (or more precisely: the blue–green distinction) seems to be caused by the introduction of a respective dye³ and not food. This suggests in turn that the new colour term started off as an identifier for that particular dye (which is in fact common, see indigo, Prussian blue, etc.). As such it doesn’t make sense that it covers the colour of something well known including food. For example, if the new colour term had covered the colour of plums, people would rather call the new dye plum dye and forget about the new term (and the dye) quickly. As a result, it is not surprising that the emerging colour term would start with a territory that excludes food. Of course this might change later in a language’s development, but if a colour starts off as a non-food colour, there is some psychological momentum to keep it that way: You are not used to eating blue stuff, hence you wouldn’t start calling food blue (see also Rumtscho’s answer).
This is paralleled in grey, the other basic colour term that we do not like to use to describe food, even though it is arguably the most fitting colour for meat in some conditions, certain fish, the inside of eggplants, etc. Rather we tend to describe these by whatever remnant hue the food has or as white or black.