Talk to any coroner and ask them if they have ever seen this in a body and I doubt if you will find one that has. Same goes for colonoscopies, in which I have NEVER seen anything even close to mucoid plaque. Even when cameras are swallowed to view the entire length of the intestine.
I did talk to one naturopathic doctor and he said the only time you are going to see something tarry like that coming out of the colon is if people are overdosing on iron salts. Which I would have to agree with this since this is very similar to what an upper intestinal bleed would look like.