building block vs key

building block and key both are nouns.

Word	Noun	Adjective	Verb	Adverb
building block	Yes	No	No	No
key	Yes	Yes	Yes	No

As nouns, key is a hyponym of building block; that is, key is a word with a more specific, narrower meaning than building block:

Other hyponyms of building block include ashlar, breeze block, cinder block, clinker block, headstone, keystone, voussoir.

building block (noun)	key (noun)
a block of material used in construction work	metal device shaped in such a way that when it is inserted into the appropriate lock the lock's mechanism can be rotated
a single undivided natural thing occurring in the composition of something else	a lever (as in a keyboard) that actuates a mechanism when depressed
	mechanical device used to wind another device that is driven by a spring (as a clock)
	the central building block at the top of an arch or vault
	pitch of the voice
	something crucial for explaining
	any of 24 major or minor diatonic scales that provide the tonal framework for a piece of music
	a generic term for any device whose possession entitles the holder to a means of access
	a list of words or phrases that explain symbols or abbreviations
	a list of answers to a test
	(basketball) a space (including the foul line) in front of the basket at each end of a basketball court; usually painted a different color from the rest of the court
	a coral reef off the southern coast of Florida
	a winged often one-seed indehiscent fruit as of the ash or elm or maple
	a kilogram of a narcotic drug

building block (adjective)	key (adjective)
	serving as an essential component

building block (verb)	key (verb)
	harmonize with or adjust to
	regulate the musical pitch of
	identify as in botany or biology, for example
	vandalize a car by scratching the sides with a key
	provide with a key