Preheat oven and make lemon cake batter according to package directions. Fold in blueberries and pour batter into a prepared 9x13 baking dish.
Bake the cake until it springs back when touched or a cake tester inserted in the middle shows no wet batter.
Let cake cool for 10 minutes, then poke holes with the handle of a wooden spoon, about 1 inch apart.
Make pudding according to package directions, BUT only whisk for 1 minute instead of the recommended 2 minutes.
Pour pudding mixture directly into holes in the poke cake, poking the holes again with the wooden spoon handle as needed to get the pudding down in the holes. You'll need to work quickly as the pudding will become thicker as you pour! Pour remaining pudding across the top of the cake and spread evenly.
Move cake to the fridge uncovered to chill for 2-3 hours.
For the whipped cream cheese topping, beat softened cream cheese with a hand mixer on medium speed until no lumps remain. Add 3 tablespoons heavy whipping cream and continue beating until well blended, about 1 minute. Add remaining heavy cream and continue beating for about 2 minutes or until it starts to thicken. Sift the powdered sugar into the mixture slowly, beating until fully incorporated as you go. Continue beating for 1-2 minutes or until stiff peaks form.
Spread the whipped cream cheese frosting over the top of the cooled cake, then cover and return to the refrigerator for 2 hours or overnight. Serve cold!
Notes
My blueberries sank in the cake, but you can prevent this by pouring half of the batter in the pan before folding the blueberries into the other half. Add the rest of the batter on top and continue as directed.