You might try some neem oil. About a tablespoon of neem oil to a quart of water, add a few drops of liquid soap to help emulsify the oil and water, and spray on your body and around your space. It's nontoxic and seems effective at warding off mosquitoes here, so maybe it will work for the black flies as well.
Neem is also good for a lot of itchy or inflamed skin conditions. My neighbor got bit by some black flies while boating on the Mississippi River and was having trouble sleeping because of the itching. She applied a little neem oil to the bites and it really helped relieve the itching.