In a medium saucepan, combine 2 cups blackberries, 0.5 cup sugar, 2 tablespoons lemon juice, and 1 tablespoon cornstarch. Stir well, bring to boil, and simmer for 15 minutes until thickened. Press through a sieve and cool completely.

Crush 1.5 cups graham crackers into fine crumbs and mix with 6 tablespoons melted butter. Press firmly into a greased 9-inch pie dish and freeze for 15 minutes.

Beat 8 ounces softened cream cheese, 0.5 cup sour cream, and 0.5 cup sugar until smooth. Fold in cooled blackberry jam and 1 cup whipped topping.

Pour the filling into the chilled crust, smooth the top, cover with plastic wrap, and refrigerate for at least 4 hours.

Garnish with fresh blackberries before serving and slice into wedges.