Sift together flour, baking powder, soda, and salt. Set aside. Cream butter and gradually beat in sugar until light and fluffy. Beat in eggs one at a time. Add flour mixture and banana alternately in three parts. Gently stir in blueberries. Turn into oiled 9 x 13 cm loaf pan. Bake at 180ºC or 50 minutes. Turn out of pan to cool.