Process biscuits in a food processor into fine crumbles. Add sugar replacement, lemon zest, oil and enough milk (the quantity of milk depends on the kind of crackers you are using) for crumbles to come together, but not to be soggy, and process again.

Transfer into an 8×8 inch baking pan. Press crumbs firmly into the bottom and slightly up the sides of the pan. Bake the crust for 10 minutes in preheated 350 F (175 C) oven.

In a bowl, mix together sugar-free condensed milk, egg yolks, lemon juice and lemon zest. The acid in the lemon juice will help the mixture thicken a little bit.

Carefully fold in 3/4 cup of blueberries (do not mix blueberries or the mixture will turn purple) and pour the filling over the graham cracker crus. Top with the remaining blueberries and bake for 15-20 minutes, or until set.

Cool to room temperature, then chill in refrigerator before serving.