I have a couple of solutions that work well. I always spray my paper cups with cooking spray before putting in the batter. Before I add my blueberries to the batter, I coat them with a tablespoon or so of flour. This prevents them from sinking to the bottom and prevents them from "bleeding" into the batter.
By Elaine from Belle Plaine, IA
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Combine flour, sugar, and baking powder in large bowl; mix well. Cut in 1/2 cup butter until crumbly, reserving 1 cup mixture for topping.
These muffins are very tasty and easy to make! They are great at meal times or as a snack. Kids love these, too!
Combine first 6 ingredients in bowl, mixing to blend. Fold in blueberries. Spoon into greased and floured muffin cups.