Preheat oven to 350°.
In a small bowl, gently mix blueberries with 1 tablespoon of the flour.
In another bowl, blend remaining 2 cups flour, the sugar, baking powder, baking soda, salt, and cinnamon.
In a third, large bowl, whisk bananas, buttermilk, butter, and egg to blend. Stir flour mixture into banana mixture just until evenly moistened; the batter will be stiff. Gently stir in blueberry mixture.
Spread batter level in a buttered 5- by 9-inch loaf pan. Sprinkle with oatmeal streusel and press in lightly.
Bake bread in preheated oven until a toothpick inserted in the center comes out clean, 60 to 70 minutes.
Let bread cool in pan on a rack for 10 minutes, then run a knife around edges to loosen. Invert bread onto a plate, remove pan, and turn over onto rack to cool completely, about 45 minutes.