Stir together first 4 ingredients in a large bowl. Gently stir in blackberries until sugar mixture is crumbly. Spoon fruit mixture into a lightly greased 11- x 7-inch baking dish.
Cut 1 piecrust into 1/2-inch-wide strips, and arrange strips diagonally over blackberry mixture. Sprinkle top with 1 tablespoon sugar.
Bake at 425° for 45 minutes or until crust is golden brown and center is bubbly. Serve with ice cream and Sugared Piecrust Sticks, if desired.