Preheat oven to 425 degrees Fahrenheit. Spray large muffin tin with non-stick spray.
Mash banana until smooth.
Add banana, oats, beaten egg, baking powder and vanilla to a bowl.
Stir until just combined.
Stir in blueberries.
Using a large spring-release scoop (or spoon) fill muffin tins with batter. I fill to the top, these muffins do not rise very much.
Bake at 425 degrees for 5 minutes, then decrease oven temperature to 375 and continue baking for 12 more minutes.
Allow muffins to cool slightly in muffin tin a few minutes. Removing them while hot will likely cause the muffins to break apart as they are very moist.