Why do blueberries change color during cooking?
The presence of anthocyanin pigments gives blueberries their blue color.
Blueberries tend to change color during cooking. Acids, like lemon juice and vinegar make the blue in blueberries turn red. On the other hand, a batter with a lot of baking soda may turn the blueberries greenish-blue.
For helpful hints on storing, using and cooking with blueberries, and even a couple of recipes, see Clemson University Cooperative Extension's "Blueberry Basics" web site at http://www.clemson.edu/extension/hgic/food/food_safety/handling/hgic3529.html