I'm fighting Azar right now and he keeps spamming Blind, even while knocked down. While I can beat him in spite of it, he always takes out Berengar in the process. Is there any way to counteract the blindness?

Not that I know of. And I never managed to keep Berengar alive, he just drops too fast.

There is a talent in the Dexterity tree "Resistance to Blinding" that helps enormously. But since you already are there, I guess it's too late to pick this.

I don't know your character build (I went mostly for swordplay/alchemy with less focus to signs) so it's hard to give you further tips.

I eventually managed to do it, but it took a lot of tries. I think I finally just went into an all-out attack on Azar so he couldn't spend time attacking Berengar. I don't remember having trouble with blinding, but it was a long time ago so I'm not sure.

Well I guess it's possible if you try hard enough. I found Berengar dying there a fitting end story-wise, considering what he had done in the past. My Geralt never got blinded by the Noonwraiths, since he found out very quickly that they are very susceptible to Aard-Stun-One-Hit-Kill, even with a low level Aard.