Preheat oven to 350 F degrees. Prepare a 9x5 inch loaf pan by greasing it with the butter and a little flour.
Rinse a cup of blueberries and sprinkle them with a bit of flour. This will help so that the blueberries won't sink to the bottom.
In a medium bowl mash the bananas with a fork. To this bowl at the oil, milk, vanilla extract, egg, sugar and whisk. Add sugar and whisk.
In another bowl mix together the flour, baking powder and salt. Mix the flour mixture with the wet mixture using a wooden spoon or spatula. Add blueberries and mix so that the blueberries are evenly distributed in the batter.
Pour batter in prepared baking loaf pan and bake for about an hour or until an inserted toothpick comes out clean.
Remove loaf from pan and let cool before slicing into it.
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.