This sweet dish will make you all warm and fuzzy inside.
Turn the oven up to 350 F. While that's heating up, whisk 1 1/2 cups sugar, cornstarch, and a pinch of salt all together in a small bowl. You'll need this a minute.
In an another bowl, whisk flour with 1 1/2 cups sugar, baking powder, and another pinch of salt. Mix the softened butter in there until it kind of looks like cornmeal, then stir in the milk until it's nice and moist. Spread the blackberries into a 9x13-inch baking dish, and spoon the moist batter evenly on top of them. Sprinkle the batter with the cornstarch mixture, then pour the boiling water over it.
Bake the dish in the oven for 40 to 45 minutes. You want the mix bubbly, and the topping golden brown.