Light a campfire and let it burn down to a bed of hot coals. You'll need about 24-26 coals total for a 12-inch Dutch oven.
In a medium bowl, combine the blackberries, 1/2 cup sugar, cornstarch, and lemon juice. Stir gently to coat the berries. Set aside.
In a separate bowl, whisk together the flour, 1/4 cup sugar, baking powder, and salt. Cut in the cold butter using your fingertips or a fork until the mixture resembles coarse crumbs. Stir in the milk just until a soft dough forms. Do not overmix.
Line a 12-inch Dutch oven with a circular piece of parchment paper. This makes cleanup much easier—trust me.
Pour the blackberry mixture into the Dutch oven and spread evenly. Drop spoonfuls of the biscuit dough over the berries. Brush the dough with a little milk for a golden top.
Place the Dutch oven over a ring of 8-10 hot coals. Arrange 14-16 coals on the lid. Cook for 40-50 minutes, rotating the oven and lid every 15 minutes for even heat. The cobbler is done when the topping is golden and the filling bubbles thickly.
Carefully remove the Dutch oven from the coals using heavy gloves or a lid lifter. Let it cool for 10 minutes before serving. Spoon into bowls and enjoy warm.