Tricks to keep Black Flies away?

I believe that escape rather than repellents are prudent. In the Boundary Waters, despite 100% DEET, our group was chewed until we appeared to have measles. The only respite was on open water, away from boggy areas, with a good breeze to blow-away the winged predators. September frosts provide excellent conditions for beautiful days of canoeing. If you had a bed-sized mosquito netting over your entire canoe, the black flies would still get to bite you. At night, I watched them crawl one-by-one through the zipper opening in our tent to eat us. A good quality tent with zipper flaps could have at least provided relief at night. Members of our group actually suffered from the poisonous effects of being bitten and retreated by a short-cut out of the Boundary Waters.