Preheat the oven to 375°F (190°C). Line a 12-cup muffin pan with paper liners or lightly grease the cups.
Whisk together the flour, baking powder, baking soda, cinnamon, and salt in a medium bowl.
In a separate bowl, mash the ripe bananas until mostly smooth.
Add the egg, sugar, melted butter, milk, and vanilla to the mashed bananas. Whisk until well combined.
Add the dry ingredients to the wet mixture and stir gently just until combined. Do not overmix.
Toss the blueberries with 1 teaspoon of flour. Gently fold them into the batter. If using frozen blueberries, add them without thawing.
Divide the batter evenly among the muffin cups, filling each about three-quarters full. Add optional coarse sugar or extra blueberries on top.
Bake for 18–23 minutes, or until the tops are lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack.
Allow the muffins to cool slightly before serving.