In a bowl, mix the flour and 2 tablespoons of sugar. Cut in the cold butter using your fingers or a fork until the mixture resembles coarse crumbs.
Add cold water, 1 tablespoon at a time, stirring until the dough just holds together. Do not overwork. Divide into 4 equal portions and flatten into discs.
Place each dough disc on a piece of heavy-duty aluminum foil (about 12x12 inches). Top with 1/2 cup of blackberries and a sprinkle of sugar.
Fold the foil over the filling to create a half-moon packet. Crimp the edges tightly to seal, leaving a little space for steam.
Place the foil packets on a campfire grate or near hot coals (not directly in flames). Cook for 15-20 minutes, flipping once halfway, until the crust is golden and the berries are bubbling.
Carefully open the foil (watch for steam) and let cool for 5 minutes before eating. Serve warm.