In the bottom of a 9×13 baking dish, pour one box of yellow cake mix.
Spread the blueberries over the top of the cake mix.
Sprinkle on 1 to 2 tablespoons of sugar (depending upon how sweet they are).
Pour 1¼ cups of water over the top of the berries and cake mix. Gently toss the berries and cake mix to allow the water to settle towards the bottom of the pan. (Sometimes, I give it a little shake too). If the berries aren’t very juicy, add just a little more water. If they are plump, a little less.
Cut the butter into 8 pieces and spread out over the top.