I think the Black Arts - the type of necromancy that Mannimarco and all those black-robed cannon fodder in Oblivion and Skyrim use - is a distorted version of Alduin's "Slen Tiid Vo" that revives dragons. Straight from the coldharbours of Molag Bal - who might be the World Eater of the last kalpa. Well, he certainly looks like a Dragon-Manbearpig on his statues, and as the Prince of Domination he matches up with Paarthurnax's statements on the nature of dragons.

Alduin's resurrection shout is like the most sublime form of necromancy...a complete rewind to before the dragon died. Thu'um is not exactly mainstream magic though, so Molag Bal created a modified version compatible with Aldmeri magicks, with his own personal touches. The Black Arts is all about domination of souls (soul-trapping), defiling the dead (undead), and corrupting Arkay's Law. It fits his sphere perfectly, and the spells seem to be perverse time magic.

I'd also include frenzy and fear spells in the black arts.