Preheat oven to 375°F (190°C). Wash and slice peaches, removing skin by blanching in boiling water for 30 seconds. Mix peaches with fresh blueberries in a large bowl. In a saucepan, combine cornstarch, brown sugar, and water. Cook until mixture thickens. Add 2 tablespoons butter and lemon juice to the thickened mixture. Stir until smooth. Pour fruit mixture into a greased 9×13 inch baking dish. In a separate bowl, combine flour, granulated sugar, baking powder, and salt. Cut in cold butter until mixture resembles coarse crumbs. Stir in milk until just combined – don’t overmix. Drop cobbler topping in spoonfuls over the fruit filling. Sprinkle with additional sugar if desired. Bake for 35-40 minutes until topping is golden brown and fruit is bubbling. Let cool for 10-15 minutes before serving.