1Preheat the oven to 350 degrees F. Spray an 8x8 inch baking dish with cooking spray. Set aside.
In a medium bowl, combine the graham cracker crumbs, melted butter, sugar, and lemon zest. Stir until graham cracker crumbs are moist. Press crumbs firmly into the bottom and slightly up the sides of prepared pan. Bake the graham cracker crust for 10 minutes. Remove from oven and allow to cool to room temperature.
2To make the filling, combine the egg yolks and condensed milk in a medium bowl. Stir in the lemon juice and lemon zest. Stir until mixture is smooth and begins to slightly thicken. Gently fold in the 3/4 of the blueberries, reserving some for the top. Pour the lemon blueberry filling evenly over the graham cracker crust then top with the remaining blueberries. Bake for 15 minutes, or until just set.
3Cool for 10 minutes at room temperature, then refrigerate until completely chilled. Cut into bars and serve.