Preheat oven to 325 F. Place butter in a standard loaf pan and place in the oven as it preheats, removing when butter has melted, 5 minutes.
Meanwhile, In the bowl of an electric mixer, beat bananas, sugar, egg, and vanilla until bananas have broken up. Add melted butter followed by flour, baking soda, and salt. Remove bowl from mixer and fold in blueberries.
Spread residual melted butter around the inside of the loaf pan to cover. Pour batter into pan, smoothing top. Sprinkle with turbinado sugar and bake 45 minutes-1 hour until tester comes out clean.
Remove from oven, let cook 10 minutes. remove from pan and continue to cool on a wire rack.