I have seen pictures here of PBs that have white markings... There was just a thread about white toes. And I saw in a litter photo two that had a patch on their chest.
http://www.germanshepherds.com/forum/ubbthreads.php?ubb=showflat&Number=738435&page=1#Post738435
http://www.germanshepherds.com/forum/ubbthreads.php?ubb=showflat&Number=738435&page=1#Post738435