Bring blackberries to boil. Add sugar and tapioca. Pour into 9x9 baking dish and cover with topping immediately. Mix topping while berries are coming to a boil.

Topping: Mix Bisquick, sugar, melted butter and 1/2 cup milk or just enough milk to mix together. Put topping on by hand. Start at edges and the rest scatter over top. Bake 15 minutes.