In a small mixing bowl or measuring cup, combine the milk and oats and set aside to soak for 20 minutes.

Preheat the oven to 375 degrees F. Spray a standard 12-count muffin pan with nonstick spray or use cupcake liners.

Whisk the flour, baking powder, baking soda, and salt together in a small bowl; set aside.

In a large mixing bowl, whisk together the melted butter and sugars until well combined. Add the eggs, mashed banana, and vanilla extract, whisking until combined.

Pour the dry ingredients into the wet ingredients, stir a few times, then add the milk-oat mixture and fold until no flour pockets remain. 

Toss the blueberries in 2 tablespoons of flour, then add them to the batter, folding evenly.

Divide the batter between the muffin cups, and fill them all the way to the top. Sprinkle with coarse sugar, if desired, then bake for 18 to 23 minutes, or until a toothpick inserted in the center comes out clean. 

Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire rack to continue cooling.