[Keeper of Dol Guldur]

Using the dead for divination, that would be the most accurate summary of the term necromancy, not really as simple as 'black magic', but as simple as the whole divination bit goes.

Divination is more than just making predictions to the future. It can be creating prophecies by supernatural means. It's also the act of discovering 'hidden, occult' knowledge. So through communication with the dead, Sauron was discovering death's secrets and other foul practices. I think, the Mouth of Sauron could also be labelled as Necromancer, through the foul crafts of elongating his own life.

If the involvement of communication with 'dead spirits' is more important to the craft, I'd say Sauron's pets - werewolves would be a good start. Fell spirits he places in his own sorts of instruments. The Barrow Wights are also the same idea, as are the stone watchers of his strongholds. Yeah, it seems like the Nazgul might be something along the lines of a 'dead spirit' also, but they came through a sort of loophole he found in the definition of 'living' things.