Blues harp is a technique of playing an ordinary harmonica which originated in the blues, not a type of harp or harmonica.

A diatonic harmonica is designed to ease playing in one diatonic scale. The main notes of the scale are the notes which are blown, and the less important notes are those which are sucked.

Here is a standard diatonic harmonica's layout in the key of C (1 blow is middle C):

This layout easily allows the playing of notes most important in C major, that of the C major triad: C, E, and G.

Blues harp subverts the intention of this design with what is "perhaps the most striking example in all music of a thoroughly idiomatic technique that flatly contradicts everything that the instrument was designed for", by making the drawn notes the primary ones (van der Merwe 1989, p.66). Since the "in" notes are more easily bent and consist of D, G, B, F, A, this allows two things:

• bent notes and blue notes, with B and D bending down and F up, and an approximation of the
• blues scale, on G: G, Bb, C, Db, F.

This resembles the tuning of the bottleneck guitar.

Source

• van der Merwe, Peter (1989). Origins of the Popular Style: The Antecedents of Twentieth-Century Popular Music. Oxford: Clarendon Press. ISBN 0193161214.