Preheat oven to 350 degrees
Spray two 9x5-inch loaf pans with non-stick cooking spray. Cut pieces of parchment paper to fit the bottoms and drape over the short ends of the pans. This will make removing the finished loaves easier.
Mash bananas in a bowl and set aside.
In a large mixing bowl, mix sugar and butter together until creamy.
On low speed, mix in mashed bananas, extract, sour cream, baking soda and salt until combined.
Add in flour a little at a time continuing to mix until well-blended.
Reserve a small handful of blueberries for the top of loaf.
In a separate bowl, place the remaining blueberries and sprinkle with the 4 tbsp. flour. Toss to coat.
Fold the floured blueberries into the batter and pour into prepared pans.
Sprinkle reserved blueberries across tops of loaves.
Bake on center rack of oven for 50-60 minutes or until a toothpick inserted in center of loaf comes out clean.
Cool in pans for 15 minutes, then, remove from pans to a cooling rack to finish cooling.