I don't know about the white mark, but my sable boy started getting his adult coat with a darker patch of fur on his tail. As more grew in his entire tail and dorsal line were the darker, thicker fur. Now all his puppy fluff is gone, but it all started with a dark patch on the tail.
Here are before and after pics.
(at 13 weeks, can see the darker band of fur on his tail)
and now at 5 months:
Sorry, looks like you'll have to right click to open the pic. It's my first post and clearly attempting pics was too much too soon. lol