Preheat the oven to 400ºF (200ºC). Grease a 2 quart baking dish.
In a large bowl, toss the nectarines with the lemon juice. Add the vanilla, brown sugar, cornstarch and cinnamon. Stir gently to combine. Fold in the blueberries.
Transfer the fruit mixture to the prepared baking dish. Bake for 10 minutes.
Meanwhile, in a medium-sized bowl, whisk together the flour, granulated sugar, baking powder and salt until well blended. Work the butter into the flour mixture with your fingertips or with a pastry blender until pea size pieces from. Stir in the sour cream and milk just until a shaggy dough forms.
Remove the baking dish from the oven. Top the fruit with 2-tablespoon-sized portions of dough. Sprinkle the dough with coarse sugar.
Return the baking dish to the oven and continue baking until the topping is deep golden brown, about 25 minutes. Let cool at least 15 minutes before serving, to allow the sauce to thicken.