black pepper vs peppercorn

black pepper and peppercorn both are nouns.

Word	Noun	Adjective	Verb	Adverb
black pepper	Yes	No	No	No
peppercorn	Yes	No	No	No

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

black pepper: pepper that is ground from whole peppercorns with husks on
peppercorn: pungent seasoning from the berry of the common pepper plant of East India; use whole or ground

Other hypernyms of black pepper include pepper.

black pepper (noun)	peppercorn (noun)
pepper that is ground from whole peppercorns with husks on	pungent seasoning from the berry of the common pepper plant of East India; use whole or ground
climber having dark red berries (peppercorns) when fully ripe; southern India and Sri Lanka; naturalized in northern Burma and Assam