Preheat oven to 350°. Pour 1/2 cup melted butter into an 8-in. square baking dish. In a small bowl, combine flour, 1 cup sugar, milk and vanilla until blended; pour into prepared dish. In another bowl, combine blackberries, remaining 1/2 cup sugar and remaining 2 tablespoons melted butter; toss until combined. Spoon over batter., Bake until topping is golden brown and fruit is tender, 45-50 minutes. Serve warm.