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.

I got to where I can keep Berengar alive consistently; you have to keep Azar away from Berengar. Keep knocking Azar down (he's not particularly resistant to Aard), or get between them.

My Geralt usually takes Resistance to Blinding on the next level-up after he learns about Noonwraiths the hard way. This does make the Azar Javed fight a bit easier.

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.