Rinse and pick through blueberries, tossing unripe berries and removing stems.
In a heavy saucepan combine 4 cups of blueberries, 1 cup of sugar, 1/4 cup of water, and 2 tablespoons of lemon juice. Bring to a boil and then turn down to low heat and simmer for 5 minutes stirring occasionally.
Sprinkle in 2 tablespoons of cornstarch and stir for a few more minutes until the sauce thickens.
Add 2 more cups of fresh blueberries and stir.
Set the blueberry sauce in the refrigerator to cool for about 30 minutes.
A few minutes before removing the berry sauce from the refrigerator melt the butter and spread it in a 9 x 13 inch baking pan.
Crush the graham crackers and spread them over the melted butter.
In a large mixing bowl cream together cream cheese, whipped topping, and sugar.
Place dropfuls of the cream cheese mixture on top of the graham cracker crust and spread evenly.
Then spread the blueberry filling mixture over the cream cheese layer.
Top the blueberry layer with 8 ounces of whipped topping and garnish with more fresh blueberries if desired. You can also dust it with a little powdered sugar.
Cover with plastic and set in the refrigerator to chill for an hour or more or until you are ready to serve it.