Stir together the first 4 ingredients: let the mixture stand 10 minutes or until sugar dissolves.
Roll out ½ the pastry into a square 1/4 inch thick; cut into 1 ½ inch strips and place on lightly greased baking sheet.
Bake the strips for 10 minutes at 425 degrees until lightly browned. Remove to wire rack to cool. Break strips into pieces.
Spoon half of the blackberry mixture into lightly greased 13x9 inch baking dish; top with pastry pieces. Spoon remaining blackberry mixture over pastry. Dot with butter or margarine (optional).
Roll remaining pastry into a square and cut into strips about 1 inch wide and 1/4 inch thick. Optional, paint very lightly with milk and sprinkle sugar [just enough so sugar sticks] and arrange in lattice design over filling.
Bake at 350 degrees for 50 minutes until golden brown. Serve with vanilla ice cream and /or blackberry syrup.
Yield: about 8 servings taking about 25 minutes to prepare and 50 minutes to cook.