Preheat your oven to 375°F (190°C) and grease your muffin pan, coat with non-stick cooking spray, or line with paper muffin liners.
In a medium bowl combine the mashed bananas, melted butter, and beaten egg. Add the sugar, lemon (juice and zest), baking soda, baking powder, and salt. Mix well.
Add the flour and mix just enough to incorporate into the wet mixture.
In a ziploc bag combine blueberries and 2 tablespoon flour, gently roll blueberries in the flour by turning the bag upside down and back up again. Gently fold flour coated blueberries into the batter, to avoid turning the batter purple.
Spoon into greased muffin tins (or muffin papers), filling about ⅔ full. Sprinkle the coarse sugar over each muffin (if desired).
Bake at 375°F (190°C) for 20 minutes or until inserted toothpick comes out clean. Remove from the oven and allow to cool 2-3 minutes in the pan before serving warm. Remove to a wire cooling rack to cool completely if the muffins will be stored for later.