Purdue University states that black fly bites can leave different types of marks, from a small puncture located at the bite site to a large swelling the size of a golf ball. Reactions to black fly bites include headache, fever, nausea and swollen lymph nodes in the neck.
About.com states that black fly bites are painful, because black flies use blade-like mouthparts to puncture the skin and feed on blood. The bites are not always large enough to be detected, but they can still be painful. Black fly bites are not known to transmit diseases.
Black flies are prone to bite humans that are wearing dark colors. However, perspiration, perfumes and highly concentrated insect repellents can be effective in repelling them.