Preheat oven to 400°F (200°C) and line a baking sheet with aluminum foil.

In a large bowl, combine flour, ½ cup of the sugar, baking powder, and salt. Incorporate the butter using a pastry cutter or your fingers until the texture becomes coarse and crumbly. Gradually stir in the boiling water just until the mixture is uniformly moistened.

In a separate bowl, dissolve cornstarch in the cold water. Add the remaining 1 cup of sugar, blackberries, and lemon juice, and stir well. Pour this fruit mixture into a cast iron skillet and heat until it reaches a boil, stirring often to prevent sticking.

Drop spoonfuls of the prepared dough evenly over the bubbling berry mixture. Place the skillet onto the foil-lined baking sheet to catch any overflow.

Bake in the preheated oven for approximately 25 minutes or until the topping turns golden brown.