Place 2 cups blackberries in pastry shell; set aside. In a saucepan, crush 1 cup berries. Add 3/4 cup water. Bring to a boil over medium heat, stirring constantly. Cook and stir for 2 minutes. Press berries through a sieve. Set juice aside and discard pulp.
In a saucepan, combine the sugar and cornstarch. Stir in remaining water and reserved juice until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in food coloring if desired. Pour half of the glaze over berries in pastry shell. Stir remaining berries into remaining glaze; carefully spoon over filling.
Refrigerate for 3 hours or until set. Garnish with whipped topping. Refrigerate leftovers.