I think he's incredibly mercurial, but I would agree to this to an extent, most especially in "In Treatment," where I thought he was just SO amazing. And in a role that would not have been readily assigned to him previously.
I suspect that early in his career, Blair may have been perhaps arrogant (or simply confident) and then later (further) termed as "difficult16d0," whether that was earned or not.
I do want to say that his performances in "L.A Law" were charming, and that I was seriously gobsmacked by how amazing he was on "In Treatment," in a difficult, charming, scary and yet sympathetic role.
Seriously. He's a brilliant actor, and I'm so happy he's still working.

I keep thinking I'm a grownup, but I'm not.