The basic 12bb pattern I know, but it's very typically played with a "chunka-chunka" rhythm where it alternates back and forth between two chord variants each beat (Is this also known as a shuffle or is that something else?)
On guitar I can play this on 2 strings and you can get away just be moving one string a tone up/down each strum, but I don't know what the underlying chords are.
Can someone explain the chords/theory?
I agree with Tim's answer, but I'd like to add that you shouldn't think of it as changing chords. What you're doing (i.e. playing this on two strings) is actually correct and it's not like 'getting away' with something, but that's all that's to it. It's a line imposed over a chord: the chord is a dominant seventh chord (i.e. in a basic blues either the I7, IV7, or V7) and the line goes from the 5th of the chord to the 6th (and it sometimes moves on to the b7th). It happens quite often that the basic harmony is static while some line is moving. See also this answer for the more general concept of a moving line over a static chord.
You are playing a root and the 5th above it. That 5th then gets changed to a 6th. It sometimes then goes up one more fret to a m7th note. As in E - E (bottom string open) with 5th string 2nd fret, 4th fret, then, if you want 5th fret, often coming back to fret 4th fret.
So, the underlying chords would be maj (minor also works, but is unusual), 6th, then b7th.
