Preheat oven to 350 degrees and stir together 1 cup of the sugar, the flour, baking powder, and salt in a large mixing bowl. Add the milk and combine. Add the melted butter and whisk until smooth.
Pour the batter into the skillet and scatter the top with all of the blackberries. Sprinkle 1/4 cup of sugar over the top.
Bake until edges are golden and crispy, 50-60 minutes. When 10 minutes remain (topping is just turning golden), sprinkle with remaining 2 tablespoons sugar.