Preheat your oven to 370 degrees F and line a muffin tin with paper liners.
In a large mixing bowl, combine the flour, baking soda, baking powder, and salt. Set aside.
In a medium mixing bowl, mash together the bananas, sugar, egg, and melted butter until combined.
Add the banana mixture to the flour mixture and stir until just combined. Gently fold in the blueberries.
Crumb Topping
In a small bowl, combine the flour, brown sugar, and cinnamon. Use a fork cut in the cold butter.
Spoon the muffin batter into the prepared muffin tins filling each cup about 3/4 full. Divide the brown sugar mixture evenly, sprinkling it over the tops of the muffins.
Bake for 18-20 minutes or until the toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let cool in the pan for a couple of minutes before transferring them to a wire rack to cool completely.