Make lemon cake according to package directions and once removed from the oven, poke holes throughout the top of the cake with the end of a wooden spoon.
While cake is baking, make the blueberry filling.
Heat a saucepan over low/medium heat.
Stir 2 cups of blueberries, sugar, lemon juice, 3 tablespoons water into pan and bring to a boil.
Let boil for 3-4 minutes, then add remaining 1 cup of blueberries. Stir.
In a small bowl, mix 4 tablespoons water and corn starch together. Add to blueberries and stir.
Continue cooking until sauce thickens. Remove from heat and let cool.
Once cooled, spread onto the top of the cake and, using the end of a wooden spoon, gently poke the filling into the holes that were previously made.
Cover with plastic wrap and place in fridge to chill (this makes frosting the cake over the blueberries easier if the cake has been chilled).
In a medium mixing bowl, mix cream cheese, butter and lemon juice until light and fluffy. Stir in lemon zest. Gradually add powdered sugar, one cup at a time, until desired consistency is obtained.
Frost chilled cake and garnish with lemon slices and blueberries. Enjoy!