Preheat oven to 350 degrees. Grease a 9 X 13" baking pan.

Combine cake mix, water, eggs, and vegetable oil in a large bowl. Mix until moistened, then beat with an electric mixer until batter is smooth, about 2 minutes.

Spread the blueberries in the bottom of the baking pan. Layer the sugar, gelatin, and marshmallows on top. Pour the cake mix evenly over the marshmallows and smooth the top.

Bake in preheated oven until a toothpick inserted in the middle comes out clean, about 50 to 55 minutes. Transfer the pan to a wire rack and let cool, about 30 minutes. Serve with whipped topping or whipped cream.