This sounds like what is commonly referred to as "rat tail".  My ferret
has had this on 3 seperate occasions.  It's due to the pores on the tail
becoming clogged (if you look closely, you can usually see black dots).
 
The treatment that works best is to use Clean & Clear facial wash for oily
skin.  Wash the tail daily for one week, then every other day for a week.
Be sure to dry the tail thoroughly.
 
 -Lisann
 
PS Wish me luck - my ferret and I go to the shelter today to pick out a
ferret buddy for him!  :-)
[Posted in FML issue 3359]