Mix butter, sugar and brown sugar together. Add eggs, buttermilk. Mix well. Add bananas, mashed. Mix together flour, soda, salt and add to banana mixture. Add blueberries--DO NOT OVERMIX!! Add walnuts.
Pour 1/2 of mixture into a regular loaf pan--greased and floured. Sprinkle 2 tablespoons of brown sugar across top of mixture. Bake at 325 degrees for 70-80 minutes. Cool in pans 10 minutes.