Does it ever explain what blinded him?
It's stated on Page 12 of the first book of the AP.
Spoiler:
Aravashnial starts the adventure with 2 hit points and is blinded. He was dangerously close to the Storm King when the demon landed after grounding the dragon, and just before Aravashnial fell, he took a lash to the face and eyes from the balor’s flaming whip. Fortunately for the elf, it was a glancing blow that did not decapitate him—but it did destroy both of his eyes and left his face a mass of horrific burns.
Must have missed it, sorry.
No worries. It was in a short paragraph near the start of the AP and easy enough to overlook. :)