Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 4 1/2x8 1/2-inch loaf pan.
Stir and coat blueberries with 2 tablespoons cake flour in a bowl.
Mix coated blueberries, oats, pecans, 1 1/2 cups plus 3 tablespoons cake flour, baking soda, and salt in a bowl.
Beat white sugar, 1/2 cup brown sugar, Greek yogurt, and applesauce together in a large bowl until light and fluffy. Add eggs, one at a time; beat after each addition. Add vanilla extract and mashed banana; beat lightly until well blended.
Stir blueberry-flour mixture into yogurt mixture until just moistened. Spoon batter into prepared pan. Cut butter into 1/4 cup brown sugar in a small bowl until crumbly; sprinkle over batter.
Bake in the preheated oven until a toothpick inserted in the center of the bread comes out clean, about 55 minutes. Cool in the pan for 15 minutes; remove from pan and cool on wire rack for 1 hour.
