Moisture. People always forget that there's a lot of liquid inside the blueberries and when it bakes it gets released into the cupcakes. That's also why it takes much longer for blueberry muffins to finish baking compared to a plain one. read more
It's possible that the muffins are rising just fine, but they're not supposed to double in size like bread or triple like pastries; you should be filling the tins at least 3 / 4 of the way up if you want tops. read more
If you're making blueberry or other berry muffins, gently fold the berries into the batter at the very end of mixing to keep the berries from breaking apart and coloring the batter. 2. Go easy on the grease. For nicely shaped muffins with rounded tops and no overhanging edges, grease only the bottoms of the muffin cups. read more