In a mixing bowl, beat the softened cream cheese with an electric mixer until smooth and creamy, about 2-3 minutes. This ensures there are no lumps in the filling.

Add the powdered sugar, lemon juice, lemon zest, and vanilla extract to the cream cheese. Beat until well combined and fluffy, about 3-4 minutes.

In a separate bowl, whip the chilled heavy cream until stiff peaks form. This will help the filling to be light and airy.

Gently fold the whipped cream into the lemon mixture using a spatula. Be careful not to deflate the whipped cream.

In a bowl, combine the fresh blueberries with sugar (if using). Let them sit for about 10 minutes to release their juices.

In a 9x13 inch baking dish, combine the graham cracker crumbs and melted butter. Mix until the crumbs are evenly coated.

Press half of the graham cracker mixture into the bottom of the dish to form a crust.

Spread half of the lemon cream filling over the crust, smoothing it out with a spatula.

Layer half of the blueberries over the lemon cream filling.

Repeat the layers with the remaining graham cracker mixture, lemon cream filling, and blueberries.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight. This allows the flavors to meld and the cake to set properly.

Once set, cut the icebox cake into squares and serve chilled. Optionally, garnish with additional blueberries or lemon zest for presentation.