Black is a mixed bag for me. In some sense he's a very unlikely celebrity because he seems like the guy that lived down the street that you used to play Advanced D&D with...the one who played the crazy rogue or cleric and really got into his RP at the table over a few beers. If he was never a gamer past or present, I'd be absolutely shocked.
IMO, he is a funny guy, but he's like that age-old phrase you see on T-shirts and hanging on a wall plaque in your uncle's bathroom; if you can't dazzle them with brilliance, baffle them with bullshit.
If you ask me, he's 1 part brilliant, 2 parts bullshit. Brilliant enough to know how to market himself. With Tenacious D he ripped through some Zeppelin-inspired tracks and launched into string of prose that sounded like they were written and rehearsed in 20 minutes, yet he assumed the mantle of rockstar. As an actor he gives us this interesting vacant wittiness that gets him on the big screen.
This is not a slam against the man, I like him, I'm just partially baffled that he got popular.