Remove plastic overwrap from blackberry cobbler. Remove white cardboard cover from foil pan, and turn up edges of pan. Place cobbler on a baking sheet.
Bake at 400° for 50 to 60 minutes or until golden brown. Cool cobbler on a wire rack.
Stir together cake mix and next 3 ingredients. Spoon batter into a lightly greased 13- x 9-inch baking dish. Spread cobbler evenly over batter.
Combine flour and next 4 ingredients; cut in cold butter with a pastry blender until crumbly; stir in walnuts. Sprinkle mixture evenly over cobbler.
Bake at 350° for 45 to 50 minutes or until a wooden pick inserted in center comes out clean.