First of all, I saw this other question in the SE sites with a good answer, but I didn't find an explanation about the blue cones specifically.
So most human beings have 3 types of cones (cells specialized in color sensing : blue, green and red).
While it may seem obvious that the red and green-sensing cones are an evolutionary advantage (given our natural environment and how we ate for millenia), for instance to locate fruits and differentiate ripeness, I can't find a clear advantage in having the blue cones. Blue doesn't seem to relate to anything like food, predators, sexuality. Am I missing an obvious utility ?
Let's try to imagine what happens if we remove blue cones :
- The sky would look gray. I don't see a survival issue there.
- We wouldn't be able to make a difference between yellow and white (white = green + red in this case). Is it a big deal ?
I have the reflex to think "if a character is widespread, there must be a good reason". It seems I should rather say "there must be an explanation".
So I'm trying to think differently, and here are other possible approaches to address this question :
- I read that dogs have only blue & yellow cones. Maybe our ancestors had only two cone types, including blue, then a third type appeared and blue stayed just because it wasn't a problem to keep it ?
- What we call visible spectrum is in fact the only part of the spectrum that can go through air and water (in the eye) without being absorbed (link). Blue cones would be there just because "hey there's something to see" ?
- For a long time we had only fires to look at, at night. No TV or screen with Q&A sites to consult. Is it possible that some of our ancestors stared for too long a hot fire, losing green and red cones ? Those with blue cones were not totally blind and managed to reproduce ! (this one is... almost a joke)
Sadly these approaches don't really convince me.
ADDITION : here is a visual example to see what happens if we remove blue information from an image (original image taken here). The image without any blue (middle) is confusing because it makes you think we could see yellow (while yellow wouldn't be different from white). That's why I added the image on the right, containing only information from red and green channels (the blue channel is replaced by min(Red,Green)). I think it illustrates well that blue isn't important, at least for vegetables and fruits !