I love blues and everytime I listen to harp (harmonica) on some Sonny Terry with Brownie McGhee for example, I really want to play Harp, the problem is how to know which Key, should I be based on the guitar scale ?
In this example : Jimi Hendrix - My Friend Harmonica is same scale played on guitar/bass ?
Playing blues on a harp, drawing is the way to go. Blowing on, say, a C harp will give you the notes of that chord - C. So the blues notes, mainly, give the chord a fourth away - G. So, for a song in G, you'd need a C harp.
To calculate what you need, know the guitar key, and count backwards 5, or more simply, forwards 4. As in, song's in A, use D. Song's in E, use A. Song's in G, need a C harp (as in the Hendrix song!).
All that's called cross harp. A lot of good players will resort to using two in a song - one in the actual key, one in cross key.
Generally, for blues, if guitar plays in C you would use an F harp. This is to get the Bb ("blue 7th"), otherwise known as Mixolydian mode. In general, you want the harp whose key is a perfect 4th above the key of the blues. So:
For a E blues, an A harp
For a D blues, a G harp
and so on.
