In a large bowl, mash the bananas. Add the buttermilk, egg, oil and vanilla.
In a separate bowl, mix the white flour, whole wheat flour, brown sugar, ground oats, baking powder, baking soda and salt.
Add the dry ingredients to the wet ingredients, and stir with large spoon just until all ingredients are incorporated.
Gently fold in the blueberries.
Scoop or spoon into muffin cups, filling 3/4 full. Sprinkle with coarse sugar or oats if desired.
Bake for 15 to 18 minutes in the preheated oven, or until the tops of the muffins spring back when lightly touched.
-To make buttermilk substitute: place 1 tablespoon of lemon juice or white vinegar in a 1-cup measure. Full with milk. Let sit on counter for 10 minutes. Stir and use as directed. 1/4 -1/2 cup of plain unsweetened Greek yogurt also works stirred into 3/4 cup milk.
-I use a 2 1/4 inch cookie scoop to fill the muffin cups. Makes the portions equal and easy to get into the muffin cups.