Star Trek 365 does indeed say "jet black."
Well, it says two different things. In D. C. Fontana's introduction, she says it made Nimoy look like "an escapee from a minstrel show" (i.e. a white performer in blackface like Al Jolson). But later, in the main text by Paula Block and Terry Erdmann, the term "jet-black" is used. Given that Fontana was directly involved with the production while Block and Erdmann are giving a second-hand account, I'd give more weight to Fontana's version.