black horehound vs stinking horehound

black horehound and stinking horehound both are nouns.

Word	Noun	Adjective	Verb	Adverb
black horehound	Yes	No	No	No
stinking horehound	Yes	No	No	No

As nouns, black horehound and stinking horehound are synonyms defined as:

black horehound and stinking horehound: ill-smelling European herb with rugose leaves and whorls of dark purple flowers

Other synonyms of black horehound include Ballota nigra, black archangel, fetid horehound.

black horehound (noun)	stinking horehound (noun)
ill-smelling European herb with rugose leaves and whorls of dark purple flowers	ill-smelling European herb with rugose leaves and whorls of dark purple flowers