
Agree with everyone above. It is very common in GSDs to have a white mark, especially on the chest, and sometimes on the tips of the toes. One of mine had a white mark on his chest. Interestingly in some GSDs the white mark doesn't appear to grow with the dog in the same way that white markings do in other breeds and so consequently in a lot cases they 'disappear' as they become adult, though you can still find the white hairs, spread widely apart if you look for them, that's what happened with my boy.
Our rescue centre's vet made the same mistake about a litter at the beginning of this year, one black puppy had a white chest patch and on that basis he pronounced them crosses. They are turning out to be full GSD! It's a common vet error.