Funny and true - a few other things that work - wind, clothing, and neck-deep gulf water. Some say they have success with part vinegar concoctions that helps keep them off.
Black flies are present when we get a north breeze at certain times of the year (mostly Fall). When present they are usually worse at the waterline area. They are attracted to stationary objects like chairs, umbrellas, and coolers. If you have a few things like this and a couple of people hanging around you are huge magnets. Walk away from the set up and the flies will not find you as easily.
While very annoying and somewhat painful, you may be able to ignore them with some success after training yourself. It helps to know that they only bite and do not have a stinger, and they don't leave any poisons or irritants in the bite so that you don't get inflamed or itchy like from a yellow fly or mosquito bite.