Yes, BLACKFLIES is a Scrabble word!

Letter values in Scrabble US: B 3, L 1, A 1, C 3, K 5, F 4, L 1, I 1, E 1, S 1.

Is it a valid word?

BLACKFLIES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: bean aphid, aphis fabae, black fly, buffalo gnat.