First, make the box cake according to the instructions. This is where you'll need the eggs and vegetable oil. Use a 9x13 baking pan for the cake and when finished allow to cool fully.
While the cake is baking and cooling, make the blueberry compote by allow the blueberries and the maple syrup to cook a bit on medium heat. Once the juices are released and the blueberries are steaming add in the cornstarch slurry and stir until thickened. Once thickened, remove from heat and stir in the lemon juice.
After the cake has cooled, poke holes in the cake using something on the thicker side like the handle of a wooden spoon. Then, top the cake with the blueberry compote you made. Finally, spread the cool whip on top and cover. Place in the fridge for a few hours. This is a great cake to make ahead.
After a few hours or the next day, garnish the cake if you'd like, cut the cake into 12 squares, serve and enjoy.