black hemlock vs mountain hemlock

black hemlock and mountain hemlock both are nouns.

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

As nouns, black hemlock and mountain hemlock are synonyms defined as:

black hemlock and mountain hemlock: large evergreen of western United States; wood much harder than Canadian hemlock

Other synonyms of black hemlock include Tsuga mertensiana.

black hemlock (noun)	mountain hemlock (noun)
large evergreen of western United States; wood much harder than Canadian hemlock	large evergreen of western United States; wood much harder than Canadian hemlock