In a stand mixer, cream butter with sugar. Add whole bananas. Mix until mashed. Add eggs, milk, sour cream & vanilla; blend well. Scrape bowl.
In a separate bowl combine flour, baking powder, baking soda, & salt. Slowly add to wet mixture until just incorporated.
Thaw frozen blueberries in the microwave for about 45 seconds until just soft, not cooked. Toss with ½ Tbsp of flour. Fold into batter.
Spray two medium loaf pans (7.5" x 3.5") or one large loaf pan (9" x 5") with cooking spray. Divide batter evenly amongst pans.
Bake at 350*F for 45-50 minutes for medium loaves or about 60-70 minutes for a large loaf. Allow to cool in the pan for 10 minutes before turning out onto a cooling rack. Slice bread once it has cooled.