The inside color of a blueberry depends on which type it is. While you will find that some blueberries are purple on the inside, there are others which are green, blue, or even clear. Many blueberries have a white center. Although blueberries may look similar on the outside, the inside color will tell you which kind they are.
Those who get blueberries from the store with a green inside color will not have to worry about not being able to eat them. They may be different from some of the blueberries you typically eat, but they are still very much edible.