How Do You Play the Blues On the Harmonica?

How do you play the blues on the harmonica?
Playing blues on the harmonica (speaking here of the 10 hole diatonic harmonica in which most people in the Western world think of it as a "harmonica") is a different technique than it was originally designed for -  it's a different approach. 

When someone just picks up a harmonica for the first time, instinctually they blow on the harmonica:
blow here, blow there, blow everywhere.
That is how the harmonica was set up originally:

Number one blow, number four blow, seven and 10 blow are the octave and root notes for the key of the harmonica that that particular harmonica is tuned in -  all blow notes there. 

However, if you think of it this way - holes 1234 blow that's a major chord; 1234 draw is a major seventh chord. 
Blues uses sevenths and seventh chords as part of a "blues thing", and a big part of the blues technique is bending notes. 

Bending a note on the harmonica is creating a note that the harmonica was not physically tuned for. 

You have 10 blow notes, 10 draw notes, a total of 20 notes, but with combinations of bending and overblowing on the harmonica you can get almost twice that -  you can get 38 notes, an incredible range on the harmonica. 

But the main change in orientation to play the blues on the harmonica is to think,  instead of a blow instrument, to think of it as mainly an inhale, draw instrument.

Your root note instead of one blow is now two draw, or the draw chord of holes 1234, that seventh chord.

That's also called second position or "crossharp". 

So, to start to play the blues on the harmonica it's important to make that change in orientation. 

If you already play the harmonica and don't play blues, the good news is that most of your techniques apply right over to the "blues side."  

Your single note technique, your breathing techniques, your moving from hole to hole is exactly the same except for mostly drawing
and you'll need to do bending, because the Blues uses the blues scale not a  do re mi fa si do scale, but a blues scale,

which has two flatted notes or "blue notes" in it -  so it's required that you bend notes to play a lot of the blues songs on the harmonica. 

There's a lot of bluesy sort of things you can do without bending notes, in fact most of the notes you play will not be bent. 

But, bending notes on the harmonica is very much an important part of the blues harmonica technique. 

The main thing, the very first thing, is to think in terms of the draw instead of the blow, to start to play blues on the harmonica.