Preheat the oven to 350 degrees (F). Line a 9" loaf pan with parchment paper so that the sides slightly overlap, then spray the parchment paper - and any exposed pan - with non-stick spray; set aside.: As the oven warms you will notice the kitchen starting to carry a gentle warmth. This preheating ensures the loaf begins rising correctly the moment it goes in, producing an even crumb and reliable texture. If you skip lining the pan, the loaf can stick and break when removed, so take the extra minute to line and spray well. A common mistake is not fully preheating, which can lead to an undercooked middle and overbrowned edges.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon; set aside.: Mixing these dry ingredients first spreads the leavening agents and spice uniformly, so you will get consistent lift and flavor in every bite. When you whisk, note the faint, dry aroma of the cinnamon and whole wheat flour, a hint the loaf will be warmly scented. If you do not whisk them well, you might end up with pockets of baking powder or soda, which can produce small bitter spots or uneven rise.
In a large bowl, beat the sugar, oil, applesauce, eggs and vanilla until combined. Stir in the mashed bananas and blueberries. Add the dry ingredients and stir until just combined.: When you mix the wet ingredients, the batter will start glossy then thicken as the eggs incorporate, creating a cohesive base. Folding in the mashed bananas introduces a sweet, fruity aroma, and the occasional pop of a blueberry will release a faint, jamlike scent. Stirring the dry ingredients in only until they disappear is important, because overmixing activates excess gluten and can make the loaf tough. A typical error here is vigorous mixing, which yields a denser final texture.
Pour the batter into the prepared loaf pan, and smooth the top. Place the sliced banana halves on top of the loaf and gently press them down slightly into the batter, if using. Bake for 55 to 65 minutes, or until a cake tester inserted in the center of the loaf comes out clean. Cool in the pan for 15 minutes before removing the loaf and transferring it to a wire rack to cool completely.: As the loaf bakes the kitchen will fill with the combined scents of cinnamon, banana, and warm grain, and you may hear subtle crackling as the top sets. The visual cue to watch for is a golden brown top that springs back slightly when touched. The internal temperature and a clean cake tester are the best checks, because ovens vary; if you pull it too early, the center will be gummy. One common misstep is opening the oven too frequently, which can drop temperature and affect rise, so only check near the lower end of the timing window.
Once cool, slice and serve!: Cooling allows the crumb to finish setting so slices hold their shape, and you will notice a gentle aroma lingering as you slice. Serve slightly warm or at room temperature, and enjoy the contrast between tender crumb and juicy bites of blueberry . If you slice while it is too hot the loaf may crumble, so allow the full cool down for clean slices.