I was wondering are blueberry muffins fattening?

Possibly: The blueberries are a tasty addition to muffins but the main ingredients are sugars, fats and eggs which can have a high overall caloric content given the size of the muffin. Consider these in the context of your overall dietary calories especially if your goal is to lose weight.

Blueberries aren't..: The healthiest part of a blueberry muffin would be the blueberries therein. Muffins by definition are not healthy, and making a muffin with blueberries or bran or apples won't magically make the muffin a healthy food. Tasty, yes, but not healthy and fattening.