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.