Mix dry cake mix (don't follow directions on the box), baking powder, flour, milk, oil, and eggs together thoroughly. Fold in the blueberries (reserving a few to sprinkle on top) very gently, only stirring a couple of times so that your batter doesn't turn blue/purple.
Pour into lined muffin cups, filling about 2/3 full, and bake for 15-20 minutes.
Makes 16-18 muffins.
Notes
The solution to sinking fruit is to toss them in a tablespoon or two of flour from the recipe's dry ingredients — just enough flour to give them a coating. Then, fold the flour-coated fruit into the batter as the very last step before baking.