First, grab a large mixing bowl. In it, whisk together the egg, milk, canola oil, and sour cream until they are smooth and well combined. Next, add the mashed bananas and mix them in thoroughly. Using very ripe bananas is key here, as they are softer and blend more easily, creating a sweeter, more flavorful base for your muffins.
Directly into the same bowl with the wet ingredients, add the all-purpose flour, sugar, baking powder, and salt. Use a wooden spoon or a spatula to gently fold the dry ingredients into the wet ones. Be careful not to overmix the batter at this stage. You want to mix only until you no longer see streaks of flour. A few lumps are perfectly fine and are the secret to a tender muffin.
Now for the star of the show. Gently fold your blueberries into the batter. If you're using frozen blueberries, it helps to toss them with a tablespoon of flour before adding them. This little trick prevents them from sinking to the bottom and stops their color from bleeding too much into the batter. Distribute them evenly for a burst of fruit in every bite.
Preheat your oven to 425°F and line a standard 12-cup muffin pan with paper liners. Fill each liner about three-quarters full with the batter. Bake for 5 minutes at 425°F, then, keeping the muffins in the oven, reduce the temperature to 375°F and bake for another 15-20 minutes. The initial high heat helps create those beautiful domed tops. You'll know they're done when a toothpick inserted into the center comes out clean.