Peel the bananas and mash in a bowl, using a fork.
Mix the flour, baking powder, baking soda, salt, and 1/2 cup brown sugar together in a large mixing bowl. Make a well in the center and add buttermilk, egg, olive oil and mashed bananas. Fold the ingredients together until just incorporated. Gently stir in blueberries.
Spoon the batter into 12 paper-lined muffin tins and sprinkle with 1 tablespoon brown sugar. Bake for 20 to 25 minutes until golden brown on top; a toothpick inserted into the center of the muffin should emerge clean. Cool in the pan for a couple of minutes, then transfer to a rack to cool completely.