Preheat oven to 400ºF. Line a 12-cup muffin tin with paper or foil liners. Combine the flours, baking powder, and salt in a medium bowl.
To make the topping, combine the brown sugar and nutmeg in a bowl.
With an electric mixer on medium speed, beat the granulated sugar and butter in a large bowl until fluffy. Beat in the egg, egg whites, and vanilla until blended. On low speed, alternately add the flour mixture and the milk, beginning and ending with the flour mixture and scraping down the sides of the bowl. Fold in the blueberries.
Evenly spoon the batter into the cups. Sprinkle with the topping. Bake until a toothpick inserted in a muffin comes out clean, about 25 minutes. Remove the muffins from the pan and cool completely on a rack.