Prepare the lemon cake according to the package directions using a two 9-inch cake pans.
While the cakes are baking prepare the blueberry lemon sauce: In a medium saucepan, add blueberries, lemon zest, lemon juice, sugar, cornstarch, and pinch of salt. Stir to combine.
Cook over medium heat, stirring occasionally, until the mixture starts to bubble and thicken, about 7 – 8 minutes (it will take about 12 – 13 minutes for frozen blueberries). Once the mixture has thickened, remove from heat and stir in the vanilla. Let blueberry mixture completely cool.
Once the cakes are baked, cool five minutes then remove from pans; let them completely cool.
Place one cake layer on serving plate and then spoon the blueberry mixture over the top of cake. Gently place the other cake layer on top of the blueberry layer.
To make the topping: Add the lemon curd and ½ cup of the cool whip to a medium bowl and whisk together until smooth. Then add another ½ cup of the cool whip and whisk again until smooth. Using a spatula, gently fold the remaining cool whip into the lemon curd mixture.
Gently spread the whipped lemon topping on just the top cake into an even layer.
Place the cake in the refrigerator for at least 1 hour before serving. Garnish with blueberries and lemon slices, if desired.
Store covered in the refrigerator.