Why aren't blueberries blue?

Regarding color:
Nothing is actually any color... It is light rays that hit our eyes, and depending on the type of object, when a light is shown on that object then into our eyes, only certain lights are absorbed into the object's surface, thus, we see light rays. Not "technically" color.

Also, you might not be eating blueberries...

I don't know what you've been eating, but if they aren't blue, they're probably unripe or not blueberries at all. Unless you're being picky and calling it an indigo-blue mix, then that's just speculation on your part.