Preheat oven to 325 degrees F.

Add the blueberries and sugar-free Maple syrup to a medium saucepan. Cook on low for 10 minutes.

Run the blueberries through a Magic Bullet or blender. Reserve 2 tablespoons of the blueberry sauce.

In a large mixing bowl stir together the sugar-free cake mix and diet 7Up. Whisk until smooth.

Use a condiment squeeze bottle or piping bag to get the blueberry sauce into the cake. Add a few squirts to the top of the cake as well.

Use a butter knife to add swirls to the cake.

Place the cake in the oven and cook according to the directions on the box.

While the cake is baking, add the cool whip and 2 tablespoons of the blueberry sauce together. Gently fold until the color is a solid light purple. Store in the fridge until ready to use.

Once the cake has cooled add the Cool Whip frosting and serve.