From what I read of your experience, Tom I would suggest two "Homespun" tapes. They are both more "advanced" than the one Roger suggested. They are way too advanced for me, and perhaps for you too, but I did pick up a few tips and licks from them that made me feel like a better guitarist.

One is "Docs Guitar". This is a collaboration between Smithsonian Folkways and Homespun tapes. I got it partly because I thought it was a historic document. Jack Lawrence's tips on backup are particularly good, but I think Doc Watson's demonstrations do make the impossible, possible.

The other one is "The Real Merle Travis Guitar" taught by his son Thom Bresh. Bresh "relaxes" the approach to Travis' style and points out where he was (intentionally) sloppy for example.

Of course neither of these are Blues, but they use a lot of similar techniques.

For learning to fingerpick the blues, alternate tuning, etc. I have found the Stefan Grossman books the most helpful. He has some basic fingerpicking ones and some some specialized ones (eg on alternate tuning).

Actually, I prefer his audio lessons, but I am an old fogey.

His web site is http://www.guitarvideos.com/.

Murray