I disagree.

Historically, the Blues (at least in this country) arose from field hollers and gospel meetings, and served to relieve the dejectedness of slavery.

Universally speaking, however, the Blues deals with troubles, pain, heartbreak, whiskey, wimmins cheatin' on their mans, mans cheatin' on their wimmins, and other things...the stuff of everyday life. In that sense the milieu never changes, and probably never will.

What changes is the way these things are manifested, through advances in technology (the electric guitar), and what's deemed socially, culturally, and politically acceptable. For example, in Hendrix's 'Hey Joe' (not stictly a blues tune, admittedly, but the lyric is suggestive of a blues theme and serves as a convenient example), the protagonist shoots his woman when he finds out that she's been cheating on him. Today, that kind of violence toward women is socially and politically unacceptable, so I'd venture that not many modern blues tunes being written these days (at least the ones I've been listening to lately) - except by 'hardliners' like T-Model Ford, who seems to be from the 'old school' and whose style hearkens back to an earlier, John-Lee-Hooker era - by the likes of Robert Cray or Keb' Mo' are so blatantly direct in dealing with the problem of infidelity. That territory has been usurped by Rap, it seems.

The experimental '60's, in which the lines that defined the various styles of music were intentionally blurred, are no longer with us, agreed. But problems that bring on a case of the blues will be with us always. And artists will continue to sing about them, utilizing whatever tools technology or culture comes up with. In that sense the Blues will continue to be expansive.

What seems to be most enduring and tireless about the Blues genre is the I-IV-V pattern. That pattern, coupled with a slower tempo than rock, in most cases, seems the most definite way to recognize a song as belonging in the Blues category. Other chord patterns that deviate markedly from I-IV-V are used occasionally and these songs can qualify as Blues, but the Blues' signature style is I-IV-V, no matter how expansive the genre becomes. Perhaps, that, too, will someday change.