How to Play Blues Harmonica
It's like this. Your standard blues progression has three main chords and 12 bars. The three main chords are the tonic, dominant, and subdominant, but we'll call them 1, 4, and 5. Each of these chords has a note on the harmonica that corresponds. Chord #1 is found at the 2 Suck hole, chord #2 is found at the 4 Blow hole, and chord #5 is found at the 4 Suck hole.
Now let's memorize the chord progression. Twelve bars, four beats each, and the chords go like this:
1 1 1 1 4 4 1 1 5 4 1 (turnaround)
So, basically, you go 2 Suck for 4 bars, 4 Blow for 4 bars, 2 Suck for 2 more bars, then 4 Suck for one, 4 Blow for one, 2 Suck for one, and one bar of turnaround which is basically a 1 Suck.
If you just play that on the harmonica you'll hear that it's a bluesy sounding chord progression.
Choosing the key...
This is important because if you're not playing in the same key as anyone else, you're going to sound quite assy. You've got your diatonic (10 holed) harp. Check the key of it, it should be written on the side or the top or the case. The key of the harmonica is the note that the 4 Blow hole plays. The blues key that you'll play in is the note that the 2 Suck hole plays. So...
With an A Harmonica, you'll play E blues.
With a C harmonica, you'll play F blues.
With a D Harmonica, you'll play G blues.
With an Eb Harmonica, you'll play Bb blues.
With an F Harmonica, you'll play C blues.
And so forth.
Actually playing something.If you just noodle around for a while, using the chord progression I listed up above as a sort of a template, you should get an idea for what notes sound good during certain chords and what notes don't. You'll probably get an idea quicker if you're playing with other musicians.
A Few NotesI don't recommend using a C Harp as the first one you learn on - several of the lower holes, the first one specifically, are very hard to play without accidentally bending the note.
E Blues is probably the easiest for guitar players to play, so an A Harp is a good one to have, whereas if you have lots of friends with trumpets and trombones, you might want an Eb Harp to play along in Bb, which is probably the easiest to play on a brass instrument.
The terminology and the original know-how came from a book called Zen and the Art of Blues Harp Blowing, by a fellow named Dave Harp. It's all you'll need to know to be a happy and (with a good deal of practice) competent blues harpist.