KMST 1.2.435 - Luminous

I believe the point is that they are suppose to be immune to Darkness(Blind) meaning the skull shouldn't even appear, even though it doesn't work on magic damage already.

They have a beginner skill that gives 20 Insight, 20 Willpower, 20 int, and immunity to Darkness

Yes, this is what I'm getting at. I think Darkness still doesn't affect mages anyway, but I have yet to test it. While I don't think it refers to something other than the status effect, I'm not sure, I do think it's a possibility, though.

Archmages had the opposing element as summons pre-bb.

The whole "you can't control opposing elements" comes from the quests to get the summons. The point of making the summons was because you can't control the opposing element, so the only way to get stronger was to make a summon that could.