When using frozen blueberries it is important not mix too much after adding adding them because the reaction of baking soda (which is alkaline) with the antioxidants (the color compounds responsible for that vibrant purple/magenta color of blueberries) will actually cause the color compounds to turn more blue and make the batter look grey since baking soda will increase the pH of the batter. To prevent this, just scatter the berries evenly over the surface of the batter and fold them in with just a few strokes. Do not let them thaw before using them.