Preheat the oven to 325 degrees. Melt butter in a 13×9 baking dish as the oven preheats.Mix 1 1/4 cups sugar with 1/2 cup flour. Add blackberries and toss. Let it sit so the sugar dissolves a bit. In a separate bowl, mix remaining 1 1/2 cups flour and 3/4 cup sugar, baking powder and salt. Add milk and whisk until smooth. Remove dish with melted butter from oven. Pour batter over melted butter. Do not stir. Spoon blackberry mixture on top of batter. Again, do not stir. Bake for one hour until cobbler is golden brown and bubbly. Serve warm. Serves well with ice cream!