It may sound obvious, but listen to as much blues as you can possibly find! All the greats (Robert Johnson, BLind Boy Fuller and the like), and everything else you can get your hands on. An authentic yet original blues 'feel' can only be developed, so get listening! It's not always just a case of learning chords - I've been exposed to blues guitar since birth (it's caused permanent mental damage, blame my father), and it's easy to tell whether the player/singer is blues by nature, or they've studied a different genre as their main interest.

Just my little bit of input!

Ruthie