pencil lead vs black lead

pencil lead and black lead both are nouns.

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

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

pencil lead: mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil
black lead: used as a lubricant and as a moderator in nuclear reactors

Other hypernyms of pencil lead include graphite, plumbago.

pencil lead (noun)	black lead (noun)
mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil	used as a lubricant and as a moderator in nuclear reactors