2. In a large bowl, combine flour, 1/2 cup sugar, baking powder and salt. Cut in butter until mixture resembles coarse crumbs. Stir in 1/4 cup boiling water until the mixture evenly moist.
3. In a separate container, dissolve cornstarch in cold water. Combine remaining sugar, lemon juice and blackberries. Transfer to a cast iron skillet and bring to a boil, stirring frequently. Drop dough by spoon into the skillet. Place the skillet on a baking sheet covered with aluminum.
4. Bake 25 minutes in the preheated oven until the crust golden.