Rinse the blackberries and pat them dry. Pour the batter into the baking dish. Sprinkle the blackberries evenly over the top of the batter. Sprinkle ¼ cup of sugar over the blackberries. Bake until golden brown and bubbly about 1 hour. When 10 minutes of the cooking time remains, sprinkle the remaining 2 tablespoons sugar over the top.