Scroll down to the thread titled, "Favorite toilet bowl cleaner? How to get rid of that ring?" There are lots of suggestions there.

Black is rather common in the toilets of manufactured homes in the mountain west. The best thing is hydrochloric acid. Sold as either 'muratic acid' a nasty concentrated product, or read the very find print on regular cleaning products and look for 'HCl' or 'hydrochloric acid'. Will probably require some searching as it has fallen from favor. Students are not taught to handle harsh products in high school laboratories anymore.