[The Squatter of Amon Rûdh]

Surely "Necromancer" is a title given to Sauron by others, who were unaware of his true identity. In that case we can separate what Tolkien knew of the word's meaning from how it was understood by those who had applied it to the mysterious sorceror of Mirkwood. Also, since nobody really knew what was going on at Dol Guldur until Gandalf's daring infiltration, it makes perfect sense that wild rumours would have abounded about commune being held with the dead and other dark practices. I'm all for "The Necromancer" just being a title: Tolkien was usually very clever at using limited character knowledge to build credibility.