Sometime black flies can be harder to deter than mosquitos. Use a repelant with a high concentration of DEET (Deep Woods Off, Cutters, etc.). Apply it often (every half hour to 45 minutes). Thats what we found worked on Yooper black flies. It should work real well on the wimpy black flies we have here below the bridge.
Type in search Black Flies, there's ton's of info. if i knew how to bring the site here i would... wear socks black flies like to attack the ankles... They can make you go insane..lol... And i find it to work fantastic to spray the underneath of the bill of your hat. That works great... Bugs Bugs Bugs man they really bug me at times... soon the fish flies will carry us away. h2o