Preheat the oven to 350 F and spray a 9 x 5" loaf pan.
In a separate bowl, use an electric mixture to beat the butter and sugars together until light and fluffy - about 2 or 3 minutes. Add the eggs, buttermilk and vanilla and beat until combined.
Add the flour mixture and beat until just combined, then use a spoon to stir in the bananas and gently fold in the blueberries.
Pour this mixture into your prepared loaf pan and bake for about 50 - 60 minutes, or until a toothpick test comes back clean.
Allow to cool for about 15 minutes in the pan before turning out and cooling completely on a wire rack.