In popular music, the minor pentatonic scale is one of the most useful scales of all. If you only play notes from this scale in the corresponding key, you will never sound wrong. The addition of the "blue note" allows the player to be even more versatile and expressive. This sequence of notes is usually called the "blues scale" and is very popular among guitarists because it provides a wonderful basis for solo playing. However, to be able to play it accurately on a standard harp, you need advanced techniques that have to be learnt first.