Funny you should mention that, MercuryMike, cuz over at the Herb Allure forum (another mercury removal forum but not from Curezone) Two members over there are getting the same reaction with Ionic foot baths. The water turns veryyyy dark swampy brown, with specks of black. Her two little kids who have never had Amalgams only make the foot baths turn a very light brown.
Certain clays also draw out impurities like that from your skin-- and I imagine if your body is trying to rid mercury through chelating, if it cant' come out through your bowel movements or urine- it will try to come out through your skin, etc.
Don't forget too- that we are loaded with so many other industrial toxins besides mercury- so whatever reason your water is turning black-- it's BETTER OUT THAN IN! :-D