My suggestion to keep black flies off is Vatika, an East Indian hair oil with coconut, olive, and some other things. The flies smother in it, so they cannot bite. You can find it at any East Indian grocery store, $4 for enough to last for years in your camping kit. It's good for your hair, and your skin, and will not eat the finish off a table, as do some of the OTC bug sprays.
Last season I learned 2 new tricks about mosquitoes, from people who stayed here. One used beer. It worked. The other used vanilla extract. I am always looking for alternatives to the poison spray in little bottles.
The other thing is to wear light coloured clothing. A white hat is always good. Leave the smelly soaps and shampoos at home.