Using fresh blueberries will keep your batter from turning blue. But blueberries are not always available, so frozen blueberries are a great option. The frozen berries will add some streaks of blue as you stir them in. As long as you do not over stir, the batter will be fine.  You can also rinse the frozen berries in cold water a few times - until the water is lighter when you drain them. Dry them very well with paper towels.