In the bottom of a 9x13 baking dish, pour in box of cake mix.
Spread the blueberries over the top of the cake mix.
Sprinkle on 1-2 Tbsp. of sugar.
Pour 1 1/4 cup 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.
If the berries aren’t very juicy, add just a little more water. If they are plump, a little less water.
Cut a stick of butter into 8 pieces and spread out over the top.
Bake at 350 degrees for 45-50 minutes.