Preheat an oven to 350 degrees F (175 degrees C). Whisk 1 1/2 cups sugar, cornstarch, and a pinch of salt together in a small bowl; set aside.
In a separate bowl, whisk the flour with 1 1/2 cups sugar, baking powder, and another pinch of salt. Mix in the softened butter until the mixture resembles cornmeal, then stir in the milk until moistened. Spread the blackberries into a 9x13-inch baking dish, and spoon the batter evenly over top. Sprinkle with the cornstarch mixture, then pour the boiling water over top.
Bake in the preheated oven until the mixture is bubbly, and the topping is golden brown, 40 to 45 minutes.
