I've not really noticed this, but there isn't any reason behind it, other than purely being part of the design of the shirt. Football teams (or soccer) seem to change their shirts every couple of years for newly designed shirts. It could be on their next shirt that the black stripe will be gone altogther.
Not exactly sure in Soccer but it could be a mark of respect for someone who die etc. In Aussie rules football, players would wear a black band around their left biceps if a fan, former player or member died.
part of the design or captain
its just a soccer thing.