black sheep vs sheep

black sheep and sheep both are nouns.

Word	Noun	Adjective	Verb	Adverb
black sheep	Yes	No	No	No
sheep	Yes	No	No	No

As nouns, sheep is a hypernym of black sheep; that is, sheep is a word with a broader meaning than black sheep:

black sheep (noun)	sheep (noun)
sheep with a black coat	woolly usually horned ruminant mammal related to the goat
a reckless and unprincipled reprobate	a docile and vulnerable person who would rather follow than make an independent decision
	a timid defenseless simpleton who is readily preyed upon