Place blueberries, sugar, lemon juice and butter in a saucepan. Bring to a boil.
Stir cornstarch into ¼ cup cold water until dissolved. When berries are boiling, drizzle the cornstarch slurry into berry sauce, stirring constantly.
Stir and cook sauce until somewhat thick. Remove from heat. It will thicken more as it cools. It needs to be cooled to room temperature before spreading on cream cheese filling.
Graham crust:
Crush graham crackers in a food processor or use a plastic bag and a rolling pin. Add soft butter and mix well with hands. Press into a 9x13 pan and chill.
Filling:
Place softened cream cheese and powdered sugar (if using sugar) into a mixing bowl Beat well with electric mixer. When smooth, add cool whip one large spoonful at a time, until all mixed together .
Assemble:
When crust is chilled, spread with cream cheese filling. Pour blueberry sauce over top and chill completely.
Notes
You can use fresh or frozen blueberries. You may also use blackberries or mixed berries and it will work just fine. Sugar in the cream cheese filling is optional. I don't use it.This makes a thick cream cheese layer, and a thin blueberry layer. You could double the berry sauce if you love berries! I love cream cheese so that is why I make that layer thicker.