system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there so few blue foods in nature?

Luke Skywalker's aunt gave him blue milk.  Where's our blue food?
gpt
Blue food is a rare occurrence in nature. There are no leafy blue vegetables (blue lettuce?), no blue meats (blueburger, well-done please), and aside from blueberries and a few blue-purple potatoes from remote spots on the globe, blue just doesn't exist in any significant quantity as a natural food color. Consequently, we don't have an automatic appetite response to blue. Furthermore, our primal nature avoids food that are poisonous. A million years ago, when our earliest ancestors were foraging for food, blue, purple and black were "color warning signs" of potentially lethal food. 


Most plant life contains a combination of chlorophyll, keratins and anthocyanins in some combination (plus other things, of course). Chlorophyll makes things appear green. Keratins make things appear red, yellow or orange. Anthocyanins have a wider scope and can reflect back reds, purples, yellows, magentas and blues. Pretty much, how an anthocyanin displays colour depends on the pH of the soil/air around the plant.