Light your campfire or charcoal chimney. You need about 25-30 hot coals for a 10-inch Dutch oven.
In a large bowl, whisk together both flours, sugar, baking powder, and salt.
In a separate bowl, beat the egg, then stir in the milk, oil, and vanilla.
Pour the wet ingredients into the dry. Stir with a fork until just combined — a few lumps are fine. Do not overmix.
Gently fold in the blackberries. Reserve a handful for the top.
Grease the inside of your Dutch oven with oil or butter. Pour in the batter and spread evenly.
Scatter the reserved blackberries on top and sprinkle with turbadino sugar.
Place the lid on the Dutch oven. Arrange coals in a ring around the bottom and pile the rest on the lid. Aim for about 6 coals underneath and 20 on top.
Bake for 30-35 minutes. Rotate the pot and lid in opposite directions every 10 minutes for even heat.
Check doneness by inserting a toothpick or skewer into the center — it should come out clean or with a few moist crumbs. The top should be golden and springy.
Carefully lift the lid away from you to avoid steam. Let the cake cool in the pot for 10 minutes before slicing. Serve warm or at room temperature.