Blueberry Buttermilk Scones
1 cup fresh blueberries 1 egg white, lightly beaten 1 tablespoon powdered sugar, or to taste 2 cups all-purpose flour 2 teaspoons baking powder 4 tablespoons white sugar, divided cooking spray ½ cup butter, cut into chunks ½ teaspoon baking soda ½ teaspoon ground nutmeg ½ teaspoon salt ¾ cup buttermilk