blackfly vs black fly

blackfly and black fly both are nouns.

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

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

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

Other synonyms of blackfly include buffalo gnat.

blackfly (noun)	black fly (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