Necromancy and nigromancy descibe the act of black magic/ spiritualism. It comes from Greek originally and laterly Latin, according to Wikipedia. The question is, is this the ancient source of the term used in modern times with a derogatory intent towards Afro American slaves and their decendents? I stumbled across the word, and it would make sense that it is in our collective psyche after international witch hunts since before Roman times. This word meaning 'dead man' which conjures moral fear of 'black magic' links neatly with Niger/Nigerian to suit the context of slavery in America at the time. Surely? Please don't mock! Am no academic!
I am also not an academic, but I am fairly well-read, including occult studies, and other than in your OP, I have never encountered the term about which you ostensibly inquire.
Aside from mentioning nigromancy in association with necromancy /ˈnɛkrɵˌmænsi/ in its definition of the latter term, Wikipedia has little to say, mentioning only that according to,
“Herbert Stanley Redgrove necromancy was one of three chief branches of medieval ceremonial magic, the others being black magic and white magic. [But] [t]his does not correspond to contemporary classifications, which often mistake "nigromancy" ("black-knowledge") with "necromancy" ("death-knowledge")” See Wikipedia LINK.
etymonline returns the following two brief results:
A.) necromancy (n.) c.1300, nygromauncy, "divination by communication with the dead," from Old French nigromancie "magic, necromancy, witchcraft, sorcery," from Medieval Latin nigromantia (13c.), from Latin necromantia "divination from an exhumed corpse," from Greek nekromanteia, from nekros "dead body" (see necro-) + manteia "divination, oracle," from manteuesthai "to prophesy," from mantis "prophet" (see mania). Spelling influenced in Medieval Latin by niger "black," on notion of "black arts." Modern spelling is a mid-16c. correction. Related: Necromantic.
B.) necromancer (n.) c.1300, from Old French nigromansere, from nigromancie (see necromancy). see etymonline LINK
So, in summation: a) The conflation of “niger” with “necro” is the result of an error which has long ago been amended; b) No relationship exists between the erroneous formulation nigromancy and the disparaging term later used to dehumanize African-Americans, past or present; c) The term nigromancy isn’t used in modern times as far as I am aware (see Janus Bahs Jacquet's Ngram link above in commentary to OP); d) negromancy does not translate as "dead man" but basically as "the act of prophecy via communion with the dead" (necro + mancy); and e) This question strikes me as disingenuous, as well as potentially divisive and incendiary, but because I value this community, I thought it prudent to respond emphatically in answer-form rather than in commentary.