Preheat oven to 350. Grease a standard loaf pan.
Peel bananas, break them up and toss them into a bowl. Mash with a potato masher (I leave some banana pieces so you get occasional nice bites of banana in the bread; don't mash until the bananas are completely liquefied).
To the bananas, add the butter, sugar, egg, and vanilla and stir. In another bowl, stir together the flour, baking soda, and salt. Fold the blueberries into the dry ingredients. Add the flour/blueberry mixture to the liquids and stir until combined. Do not over mix.
To make the topping: in a small bowl, stir together the flour, sugar, cinnamon and salt. Cut in the cold butter until the mixture resembles crumbs.
Spread batter evenly into prepared loaf pan, sprinkle the top of the loaf with the cinnamon mixture, and stud the top of the loaf with a handful of blueberries. Bake for 60-70 minutes or cooked through in the center.