Place the blackberries in a large saucepan with 1 cup of sugar. Dissolve the cornstarch in the cold water, and stir into the berries. Bring to a simmer over medium-high heat, and cook until bubbly and thickened, 3 to 5 minutes. Pour the blackberries into a 9x13-inch baking dish; set aside.
Whisk the flour with 1/2 cup sugar, baking powder, cinnamon, and salt in a mixing bowl. With a pastry cutter, cut in the cold butter until crumbly. Stir in the pecans, oats, and brown sugar until evenly blended. Stir in the hot water until evenly moistened. Drop the batter over the blackberries by the spoonful.
Bake in the preheated oven until the top is golden brown and the berries are bubbly, about 25 minutes.