Preheat oven to 350°F. Grease a 9x5-inch loaf pan or line with parchment paper.
In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
In a large bowl, whisk melted butter and sugar until smooth. Add eggs and whisk until fully combined. Stir in vanilla, mashed bananas, and sour cream.
Gently fold the dry ingredients into the wet ingredients until just combined. Do not overmix.
Toss blueberries with 1 tablespoon flour, then fold gently into the batter.
Pour batter into prepared pan. Sprinkle the turbinado sugar over the top, if using. Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely.
Notes
Tips for Best Results
Use very ripe bananas for the best flavor and moisture.
Avoid overmixing once flour is added to keep the bread tender.
If using frozen blueberries, do not thaw before adding.