For the Cake:

1. Preheat the oven to the temperature specified on the cake mix box (typically 350°F).
2. Prepare a 9×13-inch cake pan by spraying it with nonstick cooking spray.
3. Fold the zest of one lemon into the cake mix batter and pour it into the prepared pan.
4. Bake according to the package instructions (usually 34-38 minutes) until a toothpick inserted into the center comes out clean.
5. Remove the cake from the oven and allow it to cool to room temperature (about 30 minutes).

For the Filling:

1. In a mixing bowl, combine 1 1/2 cups frozen blueberries, 1/2 cup sweetened condensed milk, and the juice of 1 lemon.
2. Mash the blueberries with a potato masher or whisk until the mixture is smooth with a few whole berries remaining.
3. Using the back of a wooden spoon, poke holes evenly across the cooled cake.
4. Pour the blueberry mixture over the cake, spreading it to ensure it fills all the holes.
5. Chill the cake in the refrigerator for at least 2 hours.

For the Topping:

1. In a large bowl, combine 1 1/2 cups cold heavy cream, 4 ounces cream cheese, 1/2 cup powdered sugar, and the zest of 1 lemon.
2. Using a hand or stand mixer, whip the mixture until stiff peaks form.
3. Spread the whipped topping evenly over the chilled cake.
4. Garnish with lemon slices and additional lemon zest.