With black or very dark green tarry bowel movements promptly have your
vet check for ulcers.
 
There is an excellent article by Dr. Bruce Williams on ulcers in a past
issue of "Ferrets" magazine, and also info in the resources (archives,
etc.) listed in the critical refs in http://www.ferretcongress.org.  If
it is ulcers think Carafate as a first move and see my post today on
meds for purchasing and safety tips.
[Posted in FML issue 4605]