Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil.
Mix flour, 1 cup sugar, baking powder, and salt together in a large bowl. Cut in butter until the mixture resembles coarse crumbs. Stir in boiling water just until the mixture is evenly moist.
Dissolve cornstarch in cold water in a separate bowl. Mix in remaining 1 cup sugar, blackberries, and lemon juice. Transfer to a cast iron skillet and bring to a boil, stirring frequently. Drop in spoonfuls of dough, then place the skillet onto the prepared baking sheet.
Bake in the preheated oven until dough is golden brown, about 25 minutes.
