Black flies are trout food. They colder and cleaner the water the more black flies. In the backcountry in Maine they can swarm anything emitting CO2. DEET works to keep them from biting but they still swarm. Some people are allergic to the bites and they swell up big time. They are usually worst from mothers day to fathers day. The move out and the mosquitoes move in.
I'll take the slow moving mosquitoes any day over black flies. The worse part is the swarm when they get in your mouth, ears, nose -- pretty much every orifice and patch of exposed skin. Like you I net up or try to find some Ol' Woodsmen fly dope -- stuff smells like pine pitch, tar and moose poop, but works very well. My wife is highly allergic to them . . . probably that sensitive Nutmegger skin of her's. That said . . . I don't know if it is climate change or I am getting more tolerant, but the last few years it seems as if Black Fly Season is not as bad or as long as it use to be.