Then they aren't Black Flies. A Black Fly is incredibly tiny. They can even get through screens and are notorious for flying up your nose or into your mouth if you are a mouth breather. They do bite, but nothing that would require hospitalization.
But they do swarm in thick groups of thousands, if not millions of bugs and buzz around your head and face, driving you to the verge of insanity. I don't remember seeing them on food though, but, it might just be that I haven't paid any attention to that. Up North they were always a spring time guest. Disney used to employ natural means to keep most of the "bugs" at bay. They would ship in birds or whatever that fed on whatever insect that they were trying to control.