Preheat oven to 350°F (177ºC) and prepare 12-cup muffin pan. You can use muffin cups, line with parchment or spray with a vegetable-based non-stick cooking spray.
Whisk together dry ingredients and wet ingredients in separate mixing bowls.
Pour banana mixture in with dry ingredients and stir together until combined. Fold in the blueberries and walnuts and transfer batter to muffin tin, dividing it equally among the 12 cups.
Bake for 20 minutes until golden and a tester comes out clean. Let cool in pan for at least 10 minutes so the muffins are easy to remove (you made need to run a sharp knife around the edges to prevent sticking) then transfer to a wire cooling rack to continue cooling.