I would have posted this private but the poster was anonymous, so...
 
My jill Ninja lost the hair on the lower part of her belly last spring.
When it grew back it looked just like a bruise with hair growing out.  Maybe
it's the same with your ferret.  On Ninja it all grew back by itself so it's
probably nothing to worry about.
 
Sincerely
Maria
[Posted in FML issue 1871]