Blues Harp Terminology
First Position: Playing the harp in the key for which it's labeled. Appropriate to folk styles. Locates most of the important notes on the blows, so makes bending difficult. Hence not a blues position.
Second Position ("Cross Harp"): The basic blues mode. Plays a fifth above the labeled key (a c harp plays in g etc.), locating most of the important notes on draws.
Third Position: The second major blues mode. If you'd play g in cross, you'd play c in third: one tone down from the key you're playing in, d. Gives a haunting, minor-key sound. Often combined with a delay pedal to make this even more extreme.
Diatonic: The basic tuning of the Marine Band and similar ten-hole harps (though there are larger versions), actually quite an eccentric arrangement in which some notes are doubled and others omitted.
Chromatic: An instrument on which every sharp, flat, and whole note is available, using a switch from the whole notes to the sharps and flats almost like a piano. Larry Adler or Toots Thielmanns. The best-known use of chromatic in pop music is Stevie Wonder. More and more blues players use one, often getting an effect similar to playing a diatonic in third position.
Bullet Mic: The basic mode of amplifying a harp, cupping a Shure "Green Bullet" over the front of the harp. There are variety of vintage and new bullet mikes available from a variety of makers. Most harp players favor crystal elements.
Bend: Pulling a draw note down from its normal pitch by work in the interior of the mouth. The blues sound depends fundamentally on bends. This is what gives the harp tremendous flexibility, and one of the features (along with basic timbre) that gives it a voice-like quality. Blow notes can also be shifted; this is sometimes called an "overblow." I can't really do it. Sorry!