Sift the dry ingredients into the wet, and using a rubber spatula, stir until everything is just mixed but still clumpy. Add blueberries, and mix again.
Divide the batter between muffin liners and bake for for 18-20 minutes, or until a toothpick inserted in the center comes out clean and the tops are golden.
Don't overmix the batter. If you mix too much, the muffins will turn out tough and dense. Stop mixing the moment you see that everything is combined. Having lumps is fine!