Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil.
In a large bowl, mix the flour, 1/2 cup sugar, baking powder, and salt. Cut in butter until the mixture resembles coarse crumbs. Stir in 1/4 cup boiling water just until mixture is evenly moist.
In a separate bowl, dissolve the cornstarch in cold water. Mix in remaining 1 cup sugar, lemon juice, and blackberries. Transfer to a cast iron skillet, and bring to a boil, stirring frequently. Drop dough into the skillet by spoonfuls. Place skillet on the foil lined baking sheet.
Bake 25 minutes in the preheated oven, until dough is golden brown.