Preheat an oven to 350 degrees F (175 degrees C). Grease 18 muffin cups, or line with paper muffin liners.
Mix together mashed bananas, egg, water, and oil in a large bowl. Mix in flour, baking soda, and baking powder until mostly smooth (you will still see lumps from the banana, but that's okay!). Gently fold the blueberries into the batter.
Bake in the preheated oven until golden and the tops spring back when lightly pressed, about 15 minutes. Remove muffins from tins, and cool on a wire rack.
