The first mention of a "black stripe" that I can find on this site dates back to 2012.
Shortly after Urban Meyer became head coach, Ohio State fans noticed that incoming freshman had a very particular mark of Cain affixed to their helmets: a black stripe that transformed an iconic piece of head gear into an iconic piece of head gear with a black stripe on it.
When he got his first head coaching job at Bowling Green in 2001, Meyer started putting a stripe down the middle of the freshmen’s helmets. When they’re removed, they officially become a member of the team.