See I kinda like that. It works for me even more than the way it's explicitly made out to be a drug by something like The Magicians.
If I were to fanwank it, I'd say the effects of dark magic aren't explicitly evil. It's the controversy over it has brought about a lot of evil shit and obviously people are gonna overreach in the pursuit of power. Nobody should blame Claudia for the stuff she did with the best of intentions. I mean, now she's gone way too far, but still.
The real problem is whatever the fuck the dark arcanum is/does. Callum seemed to have the option to get in touch with it through the Key of Aaravos. But he went with the Sky arcanum instead.
Why are the mages practicing dark magic having their bodies consumed by it? I'm not saying that makes it inherently bad, but I'd be interested to see if there's actually a cool explanation for why this is dangerous.