Toss the blackberries with 1 1/4 cups sugar and 1 1/2 tablespoons flour in a mixing bowl.
Spread into a 9x9-inch baking dish, and dot with 1 1/2 tablespoons of butter. In a separate bowl, whisk 1 1/2 cups flour together with baking powder, salt, and 2 tablespoons sugar.
Cut in the softened butter until the mixture resembles cornmeal. Stir in the milk and egg until combined. Drop the batter over the berries by the spoonful.
Bake in the preheated oven until the fruit is tender and the topping is golden brown, 20 to 30 minutes.
