In a large bowl, stir together the cornstarch an 1/4 cup cold water until cornstarch is completely dissolved.
Add 1 cup sugar, lemon juice and blackberries, and combine the mixture gently but thoroughly.
Transfer to an 8-inch cast-iron skillet.
In a bowl, combine well the flour, remaining sugar, baking powder and salt.
Blend in the butter until the mixture resembles coarse meal.
Add 1/4 cup boiling water and stir the mixture until it just forms a dough.
Bring the blackberry mixture to a boil on top of the stove, stirring.
Drop spoonfuls of the dough carefully onto the boiling mixture, and bake the cobbler on a foil lined baking sheet in the middle of a preheated 400 degree oven for 20-25 minutes or until the topping is golden.