We have had experience with the same thing.  Our vet explained that
the ferrets have oil glands all over the body, including the tail and
sometimes they excrete more oil than normal.  When this happens, the
oil glands can get clogged up.  The clogged up oil glands can look like
blackheads and it can create blackheads.  It causes severe yellowing of
the tail on lighter colored ferrets and can cause hair loss.  Since we
learned that, any time we see one of our ferrets' tails looking a little
yellow or abnormal, we wash the tail with a good human facial antibiotic
soap and things seem to clear up within a few days.  It's worth a try!
 
Debbie Whorley
[Posted in FML issue 5012]