blackfly vs gnat

blackfly and gnat both are nouns.

Word	Noun	Adjective	Verb	Adverb
blackfly	Yes	No	No	No
gnat	Yes	No	No	No

As nouns, gnat is a hypernym of blackfly; that is, gnat is a word with a broader meaning than blackfly:

blackfly: small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals
gnat: any of various small biting flies: midges; biting midges; black flies; sand flies

blackfly (noun)	gnat (noun)
small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals	any of various small biting flies: midges; biting midges; black flies; sand flies
blackish aphid that infests e.g. beans and sugar beets	(British usage) mosquito