black fly vs blackfly

black fly and blackfly both are nouns.

Word	Noun	Adjective	Verb	Adverb
black fly	Yes	No	No	No
blackfly	Yes	No	No	No

As nouns, black fly and blackfly are synonyms defined as:

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

Other synonyms of black fly include buffalo gnat.

black fly (noun)	blackfly (noun)
small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals	small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals
	blackish aphid that infests e.g. beans and sugar beets