from my own experience, what you refer to as "black colon" is also called " melanosis coli" which means: it's a condition associated with chronic laxative use in which dark pigment is deposited in the lining layers of the large intestine(which is the colon). the pigment results in a characteristic dark brown to black lining of the large intestine.

it goes away sometimes if the laxative is stopped. these certain laxatives are referred as an anthroid group, which have rhubarb derivatives in it. many of these laxatives have been used for hundreds of years. in 1997 the FDA banned the use of the popular anthranoid laxative "phenolphthalein due to fears that it might be cancer causing. animal studies have shown that extremely high doses of it let to tumors in animals which may cause cancer in humans.

I'm not sure if Cascara or Senna are the ingredients that cause this pigmentation?

I was only using a health food store product that was supposed to be safer than most over the counter laxatives and had no idea it would lead up the diagnosis of this pigmentation. they say it's caused by abuse of laxatives. I wasn't abusing any laxatives and had only used it a few times to get back my balance restored because I'd just had major abdominal surgery from having a total hysterectomy and then when I was home recovering when I wound up having to go back in the hospital for a partial blockage. surgery and pain medication and not getting out of bed enough can all cause your bowels to be sluggish and cause constipation.

it was horrible. so I thought by taking the senna or cascara would be a natural alternative which my dr told me to stop taking.

they don't warn you this on the back of the bottles and I think the companies should state a warning about it.

they used to think this condition was a precursor to colon cancer, but further studies have proven that's not so.

just thought this might be helpful to you. google "melanosis coli".
maybe it will explain it better than I tried to.
good luck with everything.

L.