The Stephan Grossman workshop stuff is good. When I finished ABGU Griff hadn't released Spent Money Blues or Traveling Blues yet so I went and found Toby Walker. His stuff is intermediate to advanced and I found I've needed every ounce of what Griff taught me in ABGU to even start Toby's lessons.
Check him out at
https://www.tobywalkerslessons.com he teaches a lot of classic acoustic blues in different styles (piedmont, country, ragtime, delta) if you do decide to buy some lessons make sure to sign up for his "
members club" first as you get a discount. Also as a members club member he sends out a lesson and a lick of the month each month kind of like Griff. His methodology is different from Griff's so I had to adjust but once I got used to his teaching style I think he's great. He's very accessible by email or Skype, though I believe he is currently touring in Germany and Austria so he might be slow to respond until he gets back.
Eric