black swan vs swan

black swan and swan both are nouns.

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

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

black swan: large Australian swan having black plumage and a red bill
swan: stately heavy-bodied aquatic bird with very long neck and usually white plumage as adult

black swan (noun)	swan (noun)
large Australian swan having black plumage and a red bill	stately heavy-bodied aquatic bird with very long neck and usually white plumage as adult

black swan (verb)	swan (verb)
	to declare or affirm solemnly and formally as true
	move about aimlessly or without any destination, often in search of food or employment
	sweep majestically