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.
Bake in the preheated oven for 45 to 50 minutes.
The amount of sugar and water you use will vary on the berries themselves. Before you start, make sure you taste the berries so you add the right amount. If they’re sweet, you won’t want as much sugar. If they’re tart, you might want a little more.
It’s really important to pour the water over the top and then be sure everything is mixed. You want the berries and the water to settle into the pan before baking.
Don't make the cake batter from the box directions. Just pour the dry cake mix into the pan.
If you prefer a crunchier top, turn the broiler on for the last minute or so to brown the cake mix top.