Preheat the oven to 400 degrees. Batter: Beat 2 cups flour, sugar, baking powder, salt, nutmeg, sour cream, milk, 1/2 cup melted butter, and eggs until thoroughly mixed. Fold in blueberries. Pour into muffin tins. Crumb Topping: Cut the ¼ cup cold butter into small cubes. Mix the oatmeal, 1/2 cup flour, brown sugar, cinnamon and cubed cold butter together with pie cutter until it forms small pebbles. Sprinkle the topping mixture on top of the batter in each muffin tin. Sprinkle each with sparkling sugar. Bake the muffins for 25-35 minutes. Serve hot. *Optional: use two butter knives to form topping, if a pie cutter is unavailable.