Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Combine cake mix, water, eggs, and vegetable oil in a large bowl; beat with an electric mixer on low speed until moistened. Increase speed to medium and beat until smooth, about 2 minutes.
Spread blueberries in the bottom of the prepared baking pan. Layer sugar, gelatin powder, and marshmallows on top. Pour cake batter evenly over marshmallows; smooth the top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 50 to 55 minutes. Transfer baking pan to a wire rack; let cake cool in pan, about 30 minutes.
Spread whipped topping over the cooled cake and serve.