1 (15.25 ounce) package white cake mix (such as Pillsbury(R) Classic White)
1 cup water
1/2 cup vegetable oil
4 cups fresh blueberries
1 cup white sugar
1 (3 ounce) package strawberry-flavored gelatin
3 cups miniature marshmallows
2 cups frozen whipped topping (such as Cool Whip(R))
Preheat oven to 350 degrees F (175 degrees C). Grease a 9×13-inch baking pan.
Combine cake mix, water, eggs, and vegetable oil in a large bowl; mix until moistened. Beat with an electric mixer on medium speed until batter is smooth, about 2 minutes.
Spread blueberries in the bottom of the baking pan. Layer sugar, gelatin, and marshmallows on top. Pour cake mix evenly over marshmallows and smooth the top.
Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, 50 to 55 minutes. Transfer baking pan to a wire rack and let cool, about 30 minutes. Serve cake with whipped topping.