Preheat oven to 350°. Spray a 13x9-inch baking dish with nonstick spray.
In a large bowl, stir together 1 /4 cup sugar and cornstarch. Add peaches, tossing gently to coat. Spoon mixture into prepared baking dish.
In same bowl, whisk together flour, baking powder, salt, and remaining ½ cup sugar. Using a pastry blender, cut in butter until mixture is crumbly. (alternatively pulse dry ingredients together with butter in food processor).
Stir in blueberries. Gradually add buttermilk, stirring until dry ingredients are moistened (dough will be sticky).
Drop dough by spoonfuls over peaches. Sprinkle biscuit topping with 2 tablespoons of turbinado sugar if desired.
Bake for 45 to 55 minutes or until biscuit topping is golden brown.
Let cool for at least 30 minutes before serving.