Probably the best advice is to listen to as much blues music as possible. From an acoustic standpoint listening to people like Robert Johnson, Son House, Mississippi John Hurt, Blind Blake and Blind Lemon Jefferson will give you a great background on the original stuff. For a more modern perspective Taj Mahal, Keb Mo and Kelly Joe Phelps play with a lot of feeling - which is what blues is all about.
I had a student a few years ago that came to me proffessing that he wanted to learn to play blues. I taught him some basic chord progressions, picking patterns and blues scales and riffs. After a few weeks I asked him what kind of blues he liked and listened to. He said 'I don't really listen to any blues, I just want to learn to play it.' Hmm....
Clapton is actually very approachable, playing wise. His clear, concise style is pretty easy to hear. If you want to learn acoustic guitar blues I'd recommend listening to as many of the old guys as possible. Then go to
www.guitarvideos.com and check out what Stefan Grossman has to offer. If you study his stuff and really practice you will be able to play blues.
Rev George