black magic vs demonism

black magic and demonism both are nouns.

Word	Noun	Adjective	Verb	Adverb
black magic	Yes	No	No	No
demonism	Yes	No	No	No

As nouns, demonism is a hyponym of black magic; that is, demonism is a word with a more specific, narrower meaning than black magic:

black magic: the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world
demonism: a belief in and reverence for devils (especially Satan)

Other hyponyms of black magic include witchcraft, witchery, bewitchment, enchantment, diabolism, Satanism, obiism.

black magic (noun)	demonism (noun)
the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world	a belief in and reverence for devils (especially Satan)