Preheat oven to 350°F (175°C).
Place blackberries in a mixing bowl and toss with 1/2 cup sugar. Set aside.
In another bowl, mix flour, remaining 1/2 cup sugar, baking powder, and salt.
Stir in the milk until just combined.
Pour melted butter into a 9x9-inch baking dish.
Pour batter over the butter, but do not stir.
Spoon the blackberry mixture evenly over the batter. Do not stir.
Bake for 45-50 minutes or until golden brown and bubbling.
Serve warm with vanilla ice cream or whipped cream, if desired.