Preheat oven to 350 F. Place a 13x9 baking dish onto a cookie sheet.
Prepare the batter for the cake mix according to the box directions. Set aside.
Cut up butter and place it in the baking dish. Place baking dish in preheated oven for 3-4 minutes, or until butter has melted. Remove from oven.
Sprinkle melted butter with brown sugar and use a fork to evenly distribute. Pour blueberries on top of brown sugar and spread around in an even layer.
Pour prepared cake batter over the blueberries.
Bake in 350°F oven for 45 to 50 minutes or until a toothpick inserted in the center comes out clean. Let cool for 10 minutes in the pan, then turn out on to large flat plate or cutting board.
Notes
You can use yellow, white, French vanilla, and even lemon cake mix for this recipe. Prepare the cake mix according to package instructions (which means you will also need the ingredients listed on the back of the box).