Melt butter in a 2-quart baking dish.
Combine 1 cup sugar, flour, baking powder, and salt in a large mixing bowl. Add milk; stir well. Pour batter over butter in baking dish. Do not stir.
Combine blackberries and remaining sugar; spoon over batter. Do not stir. Bake at 350° for 1 hour or until crust is browned.