Preheat the oven to 425 degrees F (220 degrees C). Grease a 9x13-inch baking dish.

Place blackberries, 1 1/2 cups sugar, and cornstarch in a large saucepan; add water. Bring to a boil over high heat, stirring constantly until thickened, about 5 minutes. Transfer to the prepared baking dish.

Whisk flour, 2/3 cup sugar, baking powder, and salt together in a large bowl; cut in margarine with 2 knives or pastry blender until mixture is pea-sized. Stir in milk until a soft dough forms. Drop dough by spoonfuls on top blackberry mixture.

Bake in the preheated oven until golden brown, 30 to 45 minutes.