There are black and tan working dogs and bred to a sable even will produce black and tans and sables - Dean and Karla have a very very well bred litter from their good black female (as mentioned by Marsha) and a dog who was very competitive in Germany! Really nice litter which will be nice dogs!
Kway Posthorn is black and tan and some board members who have bred to him have gotten black and tan pups from sable or black females. Lots of those youngsters owned by board memebers too! Trish from Triton's Kway pups are coming along great and old enough that some are starting to get titles. Lisa had a Kway litter from a black and tan female recently, so more black and tan working line pups.
I don't usually have black and tans as I have sable or black females and usually end up breeding to sable males. But have had black and tan in 2 litters! My Ciwan x Fenja had a bi-color pup, which means Ciwan has a black and tan recessive; as did my Zender x Bianka (Basha) - 2 out of 7 - and the Enno x Basha litter - 1 out of 8..All the other litters from Basha have been predominently sable with a black (or 3).
So the production of black and tan in the working lines is not uncommon, just that there are more sables out there. In any event, if you are looking for a working pup, color - while you have a preference - should not be your main criteria, but I understand color preferences! LOL I love black GSDs, but pretty much have sables!
Lee