In Digest 38 what makes feces black is topic:
 
My turn to talk about black poop.
 
Yes, the black color of poop which is seen following gastric bleeding is
due to oxidation of the ferrous iron to ferric iron.  You can also see
black feces when taking to many iron tablets as well.  A similar process
is seen when bismuth is oxidized - it also causes black feces (we see
this in animals -or-people- taking bismuth subsalicylate - also known as
Pepto-Bismol)
 
I you are giving iron supplements to your ferrets and it turns the feces
black, you probably should back off.  Iron overload may cause iron
depostion in a number of tissues, including the liver, resulting in a
disease called hemochromatosis.  Feeding liver generally will not cause
iron overload - it is usually either iron tablets, or in some human
cultures, beer fermented in iron pots with high levels of ferrous iron.
 
With kindest regards,
Bruce H. Williams, DVM
 
Join the Ferret Health List at
http://groups.yahoo.com/group/Ferret-Health-list
 
[Further note: dark green feces can look black.  Two FHL members had these
excellent suggestions: eiher check it by dilution with water, or put it
into a plastic bag, swish it, and look at a light through it.]
[Posted in FML issue 3349]