Preheat oven to 350 F. Place blackberries in 9x13-inch baking dish. Sprinkle with tapioca and 1/2 cup sugar. In a medium bowl using a mixer, combine melted butter and 3/4 cup sugar. Add flour, baking powder, salt and milk. Pour batter over fruit and distribute evenly. Sprinkle top with 1 cup sugar. Using a sifter, sprinkle cornstarch over sugar. To obtain a crusty top, slowly drizzle 1 cup very hot water over top making sure to moisten all the sugar and cornstarch. Bake 1 hour until top is brown. Hint: to avoid spillover, increase tapioca to 2 tablespoons.