Mix the blackberries, sugar, and orange zest in a small bowl. Let stand for 20 minutes until syrupy
Preheat the oven to 375 F. In a medium-sized bowl, whisk together the flour, baking powder, melted butter, and milk.
Pour the batter into an ungreased 8-inch square baking dish
Spoon the berry mixture on top and bake for 40-50 minutes until golden brown.