I completed a whole house remodel about 1 1/2 years ago. I installed two new Kohler toilets... Cimarron models, I think. A few months ago, I noticed black "flakes" collecting at the waterline in the bowl. After a while, I figured out that black gunk is collecting at the drain holes under the rim and periodically flaking off. I have cleaned it up with cleanser and it seems to be coming back again. The water seems to be clean in the bowl and tank. I am on a city water system that provides good, clean water. I have been around for a long time and have never seen this. I might add, I am very clean and keep a clean house and bath room!