"Black colon" may refer to ischemic colitis. When there is a lack of blood supply to an organ such as the large intestine (colon), the tissues turn dark, or in the worst case, blackened, if it dies or becomes "necrotic." That is a medical emergency, and needs to be treated with immediate emergency surgery. Considering that he as a "very weak heart" and just suffered a cardiac arrest, the odds, unfortunately, are very much against him. He has a very high chance of complications or death related to the surgery. However, there is no alternative, as leaving an ischemic or necrotic colon in him has a 100% chance of causing septic shock and a very painful death.

I'm sorry to hear of your Dad's situation, and I wish you and him the best of luck in trying to get through this. Any chance he has of survival would include an emergency, high-risk operation to remove the "blackened" portions of his colon and to restore blood flow to this vital organ. The only other alternative, unfortunately, would be to refuse surgery, sign DNR papers, and make him as comfortable as possible. That decision, of course, would be determined by him if he is awake and alert and understands the situation, or, if he does not, then hopefully he has a "Living Will," outlining the type of aggressive, life-saving measures he would or would not like to undergo. These type of questions are things that are best dealt with ahead of time, when our loved ones are alert and healthy and can make their wishes (or "advanced directives") known. It also helpful for them to appoint an official "Health Care Proxy," a spouse or adult child or other relative who knows and understands their philosophies and wishes regarding health care in these life-threatening situations.

Once again--the best of luck to you and your family. Let us know how things work out.