Preheat the oven to 375°F and line a muffin tin with paper liners or lightly grease it with butter or cooking spray.
Peel your ripe bananas, add them to a medium bowl and, using the back of a fork, mash them until well combined and a puree is formed.
In a medium bowl, add the flour, baking soda, baking powder, and salt, and stir until well combined.
In a large bowl, add the mashed banana, melted butter, sugar, egg, and vanilla extract and beat until well combined and the sugar is well incorporated.
Pour the flour mixture into the banana mixture, then beat or whisk until well combined. Depending on the ripeness of your bananas, the batter may be a little thick. If you find it too thick simply add a couple of tablespoons of milk to loosen it up slightly; you want the batter to be thick but not overly gloopy.
Add blueberries to the batter and gently fold them into the mixture. (Note: If using frozen berries, add the blueberries to a small bowl, sprinkle with a tablespoon of flour, and toss until well coated in flour – this will help the blueberries from bleeding into the batter. Then add the flour-coated blueberries to the batter, leaving any excess flour behind, and fold until well combined.)
Spoon the batter across the muffin pan, dividing it evenly to make 12 muffins.
Transfer the muffin tin to the oven and bake for 18-20 minutes until a toothpick inserted into a muffin comes out clean.
Once baked, allow to cool slightly, then transfer the muffins to a wire rack and allow to cool to room temperature.
The muffins can be served immediately, stored at room temperature for 4 days, in the fridge for up to 6 days, or frozen for up to 3 months.