What would be causing mildew or mold to be growing on the inside of the toilet where the top of the water is?

We went on vacation for a week and there was black mildew/mold looking stuff growing inside the toilet bowl at the top of the water level. I cleaned the bowl before we left and cleaned it when we got back. Two days later the black stuff is back. I cleaned with Lysol with Bleach toilet bowl cleaner and dropped one of those blue Two Thousand flushes in the tank. Two days later it is back. This stuff is only in one of the toilets and is driving me nuts because I can't keep it from coming back. What causes it and is there anything I can use to keep it from coming back? We aren't not flushing the toilet or anything. I would think, if any place, it would be in my 3.5yr old toilet but it isnt. It has me baffled.