black art vs necromancy

black art and necromancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
black art	Yes	No	No	No
necromancy	Yes	No	No	No

As nouns, black art and necromancy are synonyms defined as:

black art and necromancy: the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world

Other synonyms of black art include black magic, sorcery.

black art (noun)	necromancy (noun)
the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world	conjuring up the dead, especially for prophesying
	the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world