Stir together first 4 ingredients; let mixture stand 10 minutes or until sugar dissolves.
Roll half of pastry to 1/4-inch thickness; cut into 1 1/2-inch-wide strips. Place on a lightly greased baking sheet.
Bake at 425° for 10 minutes or until lightly browned. Remove to a wire rack to cool. Break strips into pieces.
Spoon half of blackberry mixture into a lightly greased 13- x 9-inch baking dish; top with pastry pieces. Spoon remaining blackberry mixture over pastry; dot with butter.
Roll remaining pastry to 1/4-inch thickness; cut into 1-inch strips, and arrange in a lattice design over filling. Sprinkle with sugar, if desired. Place cobbler on a baking sheet.
Bake at 350° for 50 minutes or until golden. Serve with vanilla ice cream and Blackberry Syrup, if desired.