coal black vs ebony

coal black and ebony both are nouns.

Word	Noun	Adjective	Verb	Adverb
coal black	Yes	No	No	No
ebony	Yes	Yes	No	No

As nouns, coal black and ebony are synonyms defined as:

Other synonyms of coal black include jet black, pitch black, sable, soot black.

coal black (noun)	ebony (noun)
a very dark black	a very dark black
	tropical tree of southern Asia having hard dark-colored heartwood used in cabinetwork
	hard dark-colored heartwood of the ebony tree; used in cabinetwork and for piano keys

coal black (adjective)	ebony (adjective)
	of a very dark black