I use an oxy-clean/shampoo paste on my horse's tail when it gets stained. Then I condition it. Takes out just about any stain, but does dry the hair.
Of course, when I use it on his tail, it isn't close to the skin, like it might be on your dog. You might try dabbing it on, if the hair is long enough to do it without getting it on the skin, then rinse it off after a few minutes. I'm not familiar with that type of dog so I don't know how long the hair is.
Watson