Have you ever noticed the colour of blueberries in blueberry muffins which you eat? Aren’t they green? Why does this happen?
This happens when the muffin mix contains a lot of baking soda or also when the baking soda is not mixed properly in the batter. This could result in formation of some alkaline pockets throughout the batter which gets in contact with the fruits. Blueberries are rich in anthocyanin pigment especially under their skin. This pigment is very sensitive to the PH levels and generally is associated with the acidic fruits. This pigment is surrounded by the acidic environment which is rich in hydrogen. When blueberries are added to the muffin batter their skin may break allowing soda in the alkaline pockets to get in contact with the pulp and the pigment. This increases the PH level and therefore changes the colour of the blueberries from blue to green.
To avoid this, make sure that all the dry ingredients are mixed well together before adding any liquid into your cake batter.