Preheat oven to 350°. In a small bowl, cream butter and sugar until light and fluffy, 5-7 minutes. 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 dish. Sprinkle with blackberries. Pour apple juice over top. Bake until golden brown, 45-50 minutes. Serve warm. Top with ice cream or whipped cream if desired.