Blueberries are heavier than the muffin batter so they do sink down. To avoid this happening, scoop the batter (without the berries) into the muffin liners first then dot the blueberries on top of the muffins. Use your fingers or a spoon to gently push them partway into the batter. This helps to stop the blueberries bleeding into the batter and keeps them more suspended.