To make blackberry pie, combine 3 1/2 cups blackberries, 1/2 cup sugar and 1/2 cup flour. Stir, and transfer the mixture to a prepared pie shell. Sprinkle an additional 1/2 cup berries on top of the mixture, and place the top crust on the pie, crimping the edges to seal them. Brush the pie with 2 tablespoons milk, and sprinkle it with 1/4 cup sugar. Bake at 400 F for 15 minutes and 375 F for 25 more minutes.
Alternatively, begin making a blackberry lattice pie by stirring together 6 cups blackberries, 1 cup sugar, 3 tablespoons cornstarch, 1 teaspoon lemon juice and 1/2 teaspoon lemon zest. Let stand for 20 minutes, mashing occasionally. Roll out one disk of prepared pie dough, and place it in a 9-inch pie plate. Place the prepared crust in the refrigerator while you roll out a second disk of dough and cut it into 1 1/4-inch strips.
Pour the blackberry mixture into the prepared pie crust, and dot the top with 2 tablespoons butter. Arrange the strips of dough on top of the pie to form a lattice topping. Brush the top of the pie with a mixture of 1 egg and 1 tablespoon water. Place the pie on a baking sheet, and bake at 400 F for 45 to 60 minutes. Cover the crust with foil if it starts to brown.