expanse vs blank space

expanse and blank space both are nouns.

Word	Noun	Adjective	Verb	Adverb
expanse	Yes	No	No	No
blank space	Yes	No	No	No

As nouns, blank space is a hyponym of expanse; that is, blank space is a word with a more specific, narrower meaning than expanse:

Other hyponyms of expanse include acreage, land area, footprint, erasure, place, space, space, balk, baulk, plane section, section.

expanse (noun)	blank space (noun)
the extent of a 2-dimensional surface enclosed within a boundary	a blank area
a wide scope
a wide and open space or area as of surface or land or sky