Preheat the oven to 425 degrees F (220 degrees C). Coat 2 baking sheets with cooking spray.

Whisk flour, 3 tablespoons sugar, baking powder, baking soda, salt, and nutmeg together in a large bowl. Cut in butter with 2 knives or a pastry blender until mixture resembles coarse meal. Stir blueberries and buttermilk into the flour mixture until combined.

Gather dough into a ball and knead on a lightly floured board for 2 minutes. Then roll dough out into a 3/4-inch-thick slab. Cut into 3-inch triangles with a sharp knife and distribute scones evenly onto the prepared baking sheets. Brush tops with egg white and sprinkle with remaining 1 tablespoon sugar.

Bake in the preheated oven until lightly browned, about 15 minutes. Remove from the baking sheets and place on cooling racks. Dust tops with powdered sugar; serve warm.