In a mixing bowl, cream butter and sugar. Combine flour and baking powder; add to creamed mixture alternately with milk. Stir just until moistened. Pour into a greased 1-1/2 qt. baking pan. Sprinkle with blackberries. Pour juice over all. Bake at 350 degrees for 45-50 minutes or until golden brown. Serve warm; top with ice cream or cream if desired. Yield: 6-8 servings.