Light a campfire and let it burn down to a bed of hot coals. Alternatively, set up a camp stove with a cast iron skillet.
In a large bowl, gently toss the blackberries with 1/4 cup of the sugar and the cornstarch. Let sit for 10 minutes to release juices.
In another bowl, whisk together the flour, remaining 1/2 cup sugar, baking powder, and salt. Cut in the cold butter using your fingers or a fork until the mixture resembles coarse crumbs. Stir in the milk just until a soft dough forms.
Pour the blackberry mixture into a well-greased 10-inch cast iron skillet. Drop spoonfuls of the dough over the berries, leaving some gaps for the filling to bubble through.
Place the skillet over the campfire coals or on a camp stove grate over medium heat. Cover with a lid or foil and cook for 25–30 minutes, rotating the skillet halfway through, until the topping is golden and the berries are bubbling.
Remove from heat, let cool for 5 minutes, then serve warm. Drizzle with cream or a scoop of vanilla ice cream if you have it.