Extra Soft and Moist Blueberry Muffins

"These are some of the moistest blueberry muffins I've ever had. There's no buttermilk, oil, and only one-quarter cup of melted butter for the whole batch. The secret to keeping them so moist and soft is sour cream. It works wonders to produce wonderfully tender and rich muffins. I used lite (not fat-free) with success. I used frozen blueberries since they're readily available and inexpensive. Keep them frozen (do not thaw) when adding them to the batter because they run and bleed less if kept frozen. I have not tried with fresh blueberries but if you do, baking time will be reduced, but I'm not sure by how much. The muffins don't get very golden on top so don't judge their doneness by the color of the tops, although the turbinado sugar encourages a more golden color. They get better as they cool because you can really appreciate the abundance of juicy berries in every bite of these soft, fluffy, buttery muffins...."