Prepare the lemon cake according to the package directions using a 9 x 13 inch baking dish.

In the meantime, prepare the blueberry lemon sauce. In a medium saucepan, add blueberries, lemon zest, lemon juice, sugar, cornstarch, and salt. Stir to combine. Cook over medium heat, stirring occasionally, until the mixture starts to bubble and thicken (about 12 minutes for frozen blueberries, about 6-7 minutes for fresh). Remove from heat and stir in the vanilla. Set aside.

Remove the cake from the oven and allow it to cool for 5 minutes. Use the handle of a wooden spoon to poke holes into the cake that are about an inch apart. While the cake is still warm, drizzle the blueberry lemon sauce on top. Spread the sauce into an even layer over the cake ensuring some of the sauce and blueberries get down into the holes. Let the cake cool completely (about an hour).

Add the lemon curd and ½ cup of the cool whip to a medium bowl and whisk together until smooth. Add another ½ cup of the cool whip and whisk until smooth. Add the remaining cool whip to the bowl. Use a large spoon or spatula and gently fold the cool whip into the lemon curd mixture. Spread the whipped lemon topping on top of the cake into an even layer. Place the cake in the refrigerator for at least 1 hour before serving.

Store covered in the refrigerator.