Preheat oven to 350 degrees. Grease 9x9 baking dish. Whisk 1/2 cup sugar with flour and baking powder in a mixing bowl. Stir in milk and butter until a batter forms. Scrape into the prepared baking dish. Add the blackberries, and sprinkle with 1/2 cup sugar. Pour in water. Bake in preheated oven until the batter has risen and the top is golden brown, about 45 minutes to 1 hour. Cool before serving.