A black ring in a toilet is the result of a buildup of fungi in the toilet. Toilet water is more prone to the buildup of fungi because it loses its chlorine properties by constantly sitting in the toilet.
Fungi are deteriorating organisms found around the home and are often present within the air of the home. When there are many different fungi in the home, they will begin to spread to other areas. A clean toilet that is not used often will have a higher buildup of these fungi.
In order for fungi to be properly cleaned out of a toilet, bleach must be used. The bleach should be applied throughout the toilet with a toilet bowl brush, including inside the tank. It should be allowed to sit for at least 30 minutes to ensure that the toilet is properly disinfected with the bleach. It may be necessary to use three applications of bleach to remove all of the fungi.
Cleaning a susceptible toilet with bleach often will help reduce the amount of fungi that are able to build up in the toilet. The toilet lid should also be kept down whenever it is not in use to reduce spores from entering the toilet and restrict light to prevent further growth.