Preheat the Oven and Prepare the Loaf Pan:
Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
Mix the Dry Ingredients:
In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon (if using). Set aside.
Cream the Butter and Sugars:
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.
Add the Wet Ingredients:
Add the mashed bananas, eggs, and vanilla extract to the butter mixture. Beat until smooth and fully combined.
Add the Dry Ingredients to the Wet Ingredients:
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.
Add Buttermilk and Blueberries:
Stir in the buttermilk until the batter is smooth. Gently fold in the blueberries and chopped walnuts, if using.
Pour the Batter into the Loaf Pan:
Pour the batter into the prepared loaf pan, spreading it out evenly.
Bake the Bread:
Bake in the preheated oven for 60-70 minutes, or until a toothpick inserted into the center comes out clean. You may want to check the bread after 50 minutes and cover it with aluminum foil if the top is browning too quickly.
Cool the Bread:
Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.