Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a mixing bowl, mash the ripe bananas until smooth.
Add the melted butter to the mashed bananas and mix well.
Stir in the sugar, egg, and vanilla extract until combined.
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the banana mixture, stirring until just combined.
Gently fold in the fresh blueberries.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 20-25 minutes, or until a toothpick comes out clean.
Let cool in the pan for a few minutes before transferring to a wire rack to cool completely.