Preheat oven to 400 degrees F.
In a large bowl, mix the flour, 1/2 cup sugar, baking powder, cinnamon and salt. Cut in the butter until the mixture resembles coarse crumbs. Stir in the boiling water and mix until evenly moist.
In a saucepan, dissolve the cornstarch in cold water. Mix in the remaining 1 cup sugar, lemon juice and blackberries. Bring to a boil over medium to medium-high heat, stirring frequently until thoroughly mixed and all cornstarch has been completely dissolved. Pour mixture into an oven-safe baking dish.
Drop the dough by spoonfuls onto the top of the berries.
Bake 25 minutes in the preheated oven, until dough is golden brown.