In a medium bowl, combine blackberries, 1/2 cup sugar, lemon juice, and cornstarch. Stir gently to coat berries. Set aside.
In a separate bowl, whisk together flour, 2 tablespoons sugar, baking powder, and salt. Cut in cold butter using a fork or your fingers until mixture resembles coarse crumbs.
Pour in milk and stir just until a soft dough forms. Do not overmix.
Prepare your campfire or camp stove. For a Dutch oven, you need a bed of hot coals. Place the Dutch oven on a grate or directly on coals.
Pour the berry mixture into the Dutch oven. Drop spoonfuls of dough over the top, spreading slightly. Cover with the lid.
Using tongs, place 6-8 hot coals on top of the lid to create even heat. Cook for 25-30 minutes, rotating the Dutch oven a quarter turn every 10 minutes to prevent hot spots.
Check for doneness: the topping should be golden and a toothpick inserted comes out clean. Let cool for 5 minutes before serving. Enjoy warm, optionally with a scoop of vanilla ice cream.