You asked, so here it is: the reason you can blind Illidan is because he is not blind, in the common sense of blindness. His eyes were burned out, but he was given orbs of fel flame in their place that rest within his empty sockets. In a sense, think Mad-Eye Moody from Harry Potter; he has false eyes but they still allow him to see. He is only wearing a blind as to not discomfort or frighten others of what lies in place of eyes. But the blind doesn't hamper his vision. He sees all forms of magic, as well as Predator-vision, so to speak. In essence, he can now see more than he ever could. However, it can still be argued whether or not he can be blinded by provincial means. He has been shown in-game to recoil at very bright light, though, that could've been because it was literal Light in the WoW universe, which is nearly the opposite of Illidan's fel/chaos magic that fuels his very being. Furthermore, it could be said that using fire magic against Ragnaros is folly, as it either does reduced damage or simply empowers him. In the original 40-man raid of vanilla WoW, you just didn't use anyone that used fire magic, because it did less damage. Canonically, at that time, it was said to empower Ragnaros or have no effect. Later on, when he was resurrected during WoW: Cataclysm, Ragnaros had no resistance or weakness to any certain element within the game, though it can be said that he was still empowered by his own element, but this never played a role in the game except in the fact that he could only exist within his fire realm or where he'd spread his elemental plane's presence (as he slowly attempted to burn and acquire all of Mount Hyjal in this fashion). I could go on, but I won't. But I could. But I won't.
It might pain Khaldor as a dog person, but when casting the Cats, slipping "meow" into his casts instead of "now" would be the natural extension of the Misha puns or the "clawing their way back into the game"-type comments.
