[thedudedominick]

I don't know specifically about those two teams, but some teams use helmet stripes/logos as rewards for players. Sometimes it's the Freshmen that have to earn a stripe/logo, other times it's everyone.

Could also denote an injured player that no one else can make contact with but I don't know if a team would use a helmet stripe for that.

edit: just looked up Tennessee and that's exactly what it is. Freshmen have to "earn their stripes" so they start off with the black one.