black willow vs willow

black willow and willow both are nouns.

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

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

black willow: North American shrubby willow having dark bark and linear leaves growing close to streams and lakes
willow: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of black willow include willow tree.

black willow (noun)	willow (noun)
North American shrubby willow having dark bark and linear leaves growing close to streams and lakes	a textile machine having a system of revolving spikes for opening and cleaning raw textile fibers
	any of numerous deciduous trees and shrubs of the genus Salix