I just want to amplify on PG's point about the fact that the blues "old guys" didn't play or sing a piece the same way every time, because my favorite dead horse to beat on in this never ending discussion is that, once you take the music out of its home environment, no matter how fastidious you are about "authenticity", it stops being "traditional" and becomes something new.

"The Blues" as practiced by the "old guys", was a process of extemporaneous composition--they had a whole collection of couplets, lines, licks, bass runs, floating verses, rhythmic patterns, and song fragments that they used to highlight, amplify, and comment on the pieces that they were performing, depending on the responses of the audience. It was a dialog with the audience, and it spoke to a common experience, shared by the audience, who responded, on a word to word, line to line basis.

The thing is that all music (and all performing arts, for that matter) is a dialogue between the performer and the artist. We can play blues from now to Hell and Gone, but we'll never be part of the community that it came from. The best we can do is to take what we find worthwhile and portable from the tradition, and make it part of the community that we are a part of.