I have seen a few cases of this, we used to call it pseudo- 
melanosis coli, often present as a result of laxative (anthracene) 
"abuse".
It's distribution could range from the mucosa to the lamina propria, 
sub mucosa and muscularis, where the damaged, macrophage 
ingested, epithelial cells would be further digested to form a 
brown/black pigment (probably lipofuscin or similar).Usually PAS 
positive (depending on age),melanin negative. 
Terry. 
Terry Hacker,
Medical Research Council,
Harwell,
Didcot,
Oxfordshire, OX11 ORD
01235 834393 x360