You expect to see a ring around the bathtub after giving a muddy toddler a bath. A ring inside your toilet bowl, especially a blue one, could come as a surprise. The cause for the discoloration is hiding inside your water supply.
If a blue ring shows up in your toilet bowl, it will rim the bowl at the same height as the usual water level in the bowl. The color blue can range from very dark to a lighter, bluish-green color. You will probably see blue stains around the very top of the bowl as well, where water from the tank pours in. The stains will show up in every toilet in your home. If left alone, the blue ring will grow and expand, discoloring more of the toilet bowl.
The cause of the blue ring is elevated levels of copper in your water supply, which causes your water to be more acidic than is usual. The amount of copper won't affect the way the water smells or tastes, and doesn't present a health risk to you or your family. The copper stains the white porcelain used to construct your toilet bowl. The stains won't cause permanent damage, although the discoloration is unsightly and becomes more difficult to remove the longer it is allowed to sit.
To get rid of blue stains in your toilet bowl, use a mixture of 1 cup water and 1 cup ammonia. Before creating the solution, open any windows you can and turn on the exhaust fan so the bathroom is well ventilated. Put on eye protection and wear gloves, then combine the water and ammonia. Pour it into the toilet and let it stand for ten minutes. Use the toilet brush to scrub away the softened stain, then flush the toilet. Rinse the brush off in the fresh water, then flush the toilet again. Let the room continue to ventilate for another half hour.
As long as your water supply contains high levels of copper, the blue ring will eventually return to your toilet bowl. Make cleaning it out a regular part of your home maintenance routine. Every few months, use the ammonia and water solution to remove the ring and any other blue stains. This keeps them from building up enough to be an eyesore or a real chore to clean up.