The black flies in NY's Adirondack Mountains are legendary. They drive away all but the most determined outdoors men in late spring and early summer. I must say that I know them only by reputation, because I avoided those places in black fly season specifically because of those stories.
p.s. One great sounding tip I heard was to wrap fly paper around your neck and your hat when trekking in The Adirondacks in fly season. Those nasties always buzz your head and neck first before biting, and they get trapped.
Their cousin black flies in VT's Green Mountains are also very pesky, but they aren't so aggressive. We had plenty of them in our Moosalamoo camp site the past two nights. But non-aggressive does not apply not tough.
One such fly made the mistake of landing on the arm of my canvas chair. WHAM I smashed him full force with my hat. He fell over dead into the cup holder. But after a minute he revived and flew away. Very impressive. I imagined him lying there saying "Oh my freaking head." just like Superfly in this old-time video by Joe Cartoon (very funny)