[The Squatter of Amon Rûdh]

Two definitions of the word "necromancy" have been given above, along with the meanings of nekros and niger. Here is a repetition of the etymology, from my 1951 Concise Oxford (I have expanded most of the abbreviations): [Old French nygromancie, from medieval Latin nigromantia changed by association with Latin niger black from Latin from Greek nekromanteia (prec., -MANCY)]. The prefix necro-, as mentioned above, is a modification of the Greek word nekros ("corpse", "dead body").

how did Gandalf not realize it was Sauron, even though he'd been into the dungeons of Dol Goldur (where he got the key from Thrain)?

But he did realise:

Gandalf again enters Dol Guldur, and discovers that its master is indeed Sauron, who is gathering all the rings and seeking for news of the One, and of Isildur's heir, He finds Thráin and receives the key of Erebor. Thráin dies in Dol Guldur.

Entry for the year 2850 of the Third Age in the Tale of Years.

Some here will remember that many years ago I myself dared to pass the doors of the Necromancer in Dol Guldur, and secretly explored his ways, and found thus that our fears were true: he was none other than Sauron, our Enemy of old, at length taking shape and power again.

Gandalf at the Council of Elrond

My interpretation of this is that when Gandalf uses the term "Necromancer" he does so in order more readily to identify a known personage to others. He is himself fully aware of Sauron's real identity, but he uses the title for the benefit of those to whom "The Necromancer" might be a more familiar name. In the example above his motive is different: he uses the title so that he may then give it to Sauron, since there are those present at the Council who may be unaware that they are one and the same. To my mind there is no indication in his words that he genuinely believes Sauron to be a practitioner of necromancy, so yes: Tolkien probably did use it simply because it sounds scary.

[ April 21, 2003: Message edited by: The Squatter of Amon Rûdh ]