Yes, the previous poster has some good insight especially in your case it is probably dessicated stool stuck in the bowel especially if you have had a history of constipation. Especially if you have some yellow with the black or grey coating also indicates just a severe impediment of digestion wich you are taking care of. You can tell more about where the blockage is by the appearance of coating and where the black is.
anyway colon therapy would probably be benefical but do the internal such as the colosan. My advice just pay attention to how you feel and get help with areas you can't do yourself sometimes we all need a helping hand.