black hemlock vs hemlock tree

black hemlock and hemlock tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
black hemlock	Yes	No	No	No
hemlock tree	Yes	No	No	No

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

black hemlock: large evergreen of western United States; wood much harder than Canadian hemlock
hemlock tree: an evergreen tree, could refer to any one of a genus of conifers named Tsuga

Other hypernyms of black hemlock include hemlock.

black hemlock (noun)	hemlock tree (noun)
large evergreen of western United States; wood much harder than Canadian hemlock	an evergreen tree, could refer to any one of a genus of conifers named Tsuga