As far as I know, humans can distinguish between 5 basic tastes based on various molecules in food and their interactions. There's a level to all 5 so there can be an endless variety of tastes, and we can recognize them with some precision. What surprises me is that we can often distinguish the original flavors in a mix - for example in a salad or even more so in a blended juice. Shouldn't we perceive an entirely new flavor having the average of each basic taste? Not entirely sure this belongs on a biology forum, but I hope someone can explain :)
You're talking about the difference between "taste" and "flavor." In common speech we use these words synonymously, but they actually mean different things.
Taste refers to the five sensations your tongue can detect -- sweet, sour, salty, bitter and umami. There's growing evidence for a sixth taste, "fat."
Flavor refers to the combination of several factors:
1) taste, as just described,
2) the aromas of esters, flavinoids, ketones, and other compounds in your food, detected by the nose. This happens a) before you take a bite, and b) as you chew and swallow, releasing different compounds and higher concentrations into your nose through your retronasal passage,
3) physical sensations such as texture, heat and cold, visual appearance and even sound (for example, the crunch of celery or potato chips)
4) chemesthetic effects, like the astringency of tannins or the "artificial" heat of chili peppers. (The compound responsible for chili heat actually down-regulates physical heat receptors so they're more sensitive. Menthol up-regulates them to create a cooling sensation.)
5) Emotional and social factors, like the memory of your grandmother's homebaked bread, or many people's aversion to raw fish.
All of this put together is why blackberries and strawberries are both sweet and tart, yet blackberries taste like blackberries and strawberries taste like strawberries.
Hope that helps!
-- The collected works of Karen Page & Andrew Dornenburg
-- "Tastebuds And Molecules," Francois Chartier
-- LOTS of personal empirical evidence (I'm a chef)