blackflyPronunciation: /ˈblakflʌɪ/Definition of blackfly noun (plural same or blackflies) 1a black or dark green aphid which is a common pest of crops and gardens. Several species in the family Aphididae, in particular Aphis fabae 2 (also black fly) a small black fly, the female of which sucks blood and can transmit a number of serious human and animal diseases. Family Simuliidae: Simulium and other genera blackfly in other Oxford dictionaries Definition of blackfly in the US English dictionary