If you use frozen blueberries, I recommend gently rinsing and draining them a few times prior to use. Use lukewarm water which will help to both thaw the blueberries, and remove from the intensely colored juice that can turn your muffins purple. If you bake your muffins with your blueberries still frozen, they will likely require additional baking time. I usually rinse mine until the water runs clear, then sit on a paper towel to dry prior to adding to the muffin batter.