Preheat oven to 375° F. Line 2 muffin tins with paper liners, or use a non-stick pan and spray with non-stick spray (if you don't have liners). If you only have 1 muffin tin just do 1 batch at a time.
In a medium sized bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large bowl, mash the bananas with a fork until creamy (they will be lumpy). Add in the sugar, egg, melted butter, and vanilla. Mix until combined. Add the dry ingredients and gently mix until there are no streaks of flour left (be careful not to over mix).
Toss the blueberries in a spoonful of flour to coat them, and then gently fold them into the batter.
Scoop batter into paper liners, filling about ½-¾ of the way full. If you are topping with streusel, sprinkle 2-3 tsp of streusel on top of each muffin.
Bake for 20-22 minutes, until the edges turn golden brown and a toothpick inserted comes out clean. I usually start checking my muffins around 15 minutes just to be careful. Let the muffins cool in the pan for 5 minutes before transferring to a wire cooling rack.