To prepare filling, combine 1 cup blueberries, 1/2 cup sugar, juice, and cornstarch in a small saucepan. Bring to a boil; reduce heat, and simmer 5 minutes or until slightly thick. Place in a large bowl; add remaining 2 cups blueberries and blackberries, stirring to coat. Cover and chill.Bake frozen pie crust as directed. Place whipped topping in bottom of crust. Add berry mixture. Chill in fridge uncovered.