In a large bowl, cream butter and 1-1/4 cups sugar. Add eggs, one at a time, beating well after each addition. Combine the flour, baking powder and salt; gradually stir into creamed mixture alternately with milk. beating well after each addition. Fold in blackberries.
Fill greased or paper-lined muffins cups two-thirds full. Sprinkle with remaining sugar. Bake at 375° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from pans to wire racks. Serve warm.