black lead vs plumbago

black lead and plumbago both are nouns.

Word	Noun	Adjective	Verb	Adverb
black lead	Yes	No	No	No
plumbago	Yes	No	No	No

As nouns, black lead and plumbago are synonyms defined as:

black lead and plumbago: used as a lubricant and as a moderator in nuclear reactors

Other synonyms of black lead include graphite.

black lead (noun)	plumbago (noun)
used as a lubricant and as a moderator in nuclear reactors	any plumbaginaceous plant of the genus Plumbago
	used as a lubricant and as a moderator in nuclear reactors