Preheat oven to 375°F. Grease a 9x5x3” loaf pan with butter and coat it with sugar. (You do this like you would grease and flour a pan. Grease it first, then add about 2 tablespoons sugar to the pan and move the pan side to side until the bottom and sides are coated with sugar. Do NOT substitute cooking spray for the butter. You can skip the sugaring and just use cooking spray, if you wish.)
Cream butter and 3/4 cup sugar with a hand mixer. Set aside.
Add bananas, eggs, milk, vanilla, salt, and baking soda to a blender jar and blend until smooth.
Pour half the banana mixture into the butter mixture with 1 cup of flour. Mix with hand mixer until just incorporated, then add the remaining banana mixture and flour. Mix until just incorporated. Stir in blueberries. (Make sure that your frozen berries are not overly wet; pat them dry and/or remove any ice crystals or your batter will be too wet.) Pour into prepared pan.
Combine 1/4 cup sugar and cinnamon in a small bowl. Sprinkle evenly over the top of the batter.
Bake for 45-50 minutes until a toothpick comes out with just a few crumbs. The edges will be a dark brown and there will be a nice crack down the center.
Cool completely before removing loaf from pan, but you can cut slices from the pan after it’s cooled for about 15-20 minutes.