I believe there were several shelties registered as black and tan, however there is a question mark as to whether this was a crofter's way of describing a tricolour - i.e it was black, tan and white as opposed to black and white or brown (sable) and white. Certainly if you look at the pictures of the original island dogs, black does seem to be the prevailing colour but it is very difficult to tell from those early, grainy photographs exactly what colour the dog was.
In aiming to fix the desired collie type into the shelties, very early on the black and tan colouring disappeared - it was obviously not present in the desirable lines and some pretty intensive inbreeding was being done to those lines. I can't recall the last time a black and tan sheltie was registered but I think it was prior to the Second World War, so a very long time ago.
I think now, the chances of coming across a true black and tan sheltie are non existant. Unlike some breeds where there remains a seperate 'working/native population', all shelties world wide are descended from the same nucleus of dogs. I have never heard of a distinct line which has been maintained outside of the registered population and indeed, such a line would by now have become so inbred it would be in severe difficulties. It is really rather odd that the colour is still included in the standard as there have been quite a number of revisions since it became 'extinct.'
A glimmer of hope though is that from time to time, shelties are bred with very little in the way of white. I knew one little tri who only had a small patch of white on his chest and white toes. Perhaps if you put the word out that you are looking for a tri with as little white as possible you might get lucky, providing that is your only criteria! As with many show breeds, plain animals are not particuarly sought after and the flashier dogs carrying the classic white markings tend to be more sucessful. Most pet owners also prefer the 'Lassie look' so there is little reason for anyone to specifically breed for reduced quantities of white. I'm sure I have read somewhere though of someone 'experimenting' with very plain tri colours. I could be wrong but it might have been in the online Global Sheltie Magazine?
If you're not in any particular hurry, when I have a moment I will look through my 'archive' and see if I can turn up anything else for you - as it is an interesting subject. In the meantime I can also thoroughly recommend the 'Bowling site' and these two as well for the genetics:
http://website.lineone.net/~a.j.mcknight/Sheltie coat colour genetics.htm#The
http://www.athro.com/evo/gen/sheltie.html
(Unfortunatly the link to 'shelties without white markings' is broken)
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I would have placed it here)