Divide pastry into thirds. Roll one portion into an 8-inch square on a lightly floured surface; place on a baking sheet. Bake at 450° for 10 minutes or until lightly browned. Cool.
Roll another portion of pastry to 1/8-inch thickness on a lightly floured surface; fit into an 8- inch square baking dish.
Place half of blackberries in prepared baking dish. Top with 1/2 cup sugar and 1 tablespoon flour; dot with 1 tablespoon butter. Cover with baked pastry square, remaining blackberries, sugar, flour, and butter.
Roll remaining pastry to 1/8- inch thickness; cut into fourteen 10- x 1/2-inch strips. Arrange strips in a lattice design over blackberries. Press ends gently to seal strips to pastry overhang; trim edges and flute. Bake at 450° for 10 minutes. Reduce heat to 350°; bake 45 minutes or until golden brown. Serve warm with ice cream, if desired.