Preheat the oven to 425°F. Grease the wells of a 12-cup muffin pan, or line with papers and grease the inside of the papers., Combine the dry ingredients in a mixing bowl., In a second bowl, beat together the starter, milk, egg, melted butter, and sweetener. Blend the wet ingredients with the dry, taking about 20 seconds. Gently stir in the blueberries just until blended., Fill the cups of the prepared pan two-thirds full; sprinkle the tops of the muffins with sugar., Bake the muffins for 25 minutes, until a toothpick inserted in the center comes out clean. Remove the pan from the oven and allow the muffins to cool for 5 minutes before removing them from the pan. Don't let them cool in the pan, or they'll steam and the outside will become tough.