Preheat oven to 375°F (190°C). Lightly grease an 8x8 inch baking dish or a similar 2-quart casserole dish.

Prepare the fruit filling: In a large bowl, gently toss the sliced nectarines and 2 cups of blueberries with the 0.5 cup sugar, cornstarch, lemon zest, and cinnamon until evenly coated. Pour the mixture into the prepared baking dish.

Prepare the topping: In a separate medium bowl, whisk together the flour, 0.5 cup sugar, baking powder, and salt.

Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Stir in the 0.5 cup of fresh blueberries.

Pour the milk into the flour mixture and stir just until a thick, shaggy dough forms. Do not overmix. Drop spoonfuls of the topping evenly over the fruit mixture, leaving small gaps for steam to escape.

Bake for 40 to 45 minutes, or until the topping is golden brown and the fruit filling is bubbly and visibly thickened around the edges. If the topping browns too quickly, loosely tent with foil.

Let the cobbler cool for at least 15 minutes before serving. Serve warm, ideally with vanilla bean ice cream or fresh whipped cream.