If you have a blue stain inside your toilet bowl, do not blame a family member. The culprit is hidden inside the water that fills the toilet bowl: high levels of copper.
Blue and blue-green stains in your toilet bowl are caused by copper. Acidic water leeches copper from your pipes, and deposits it at the drain openings at the top of your toilet bowl. If left untreated, the stain will grow larger over time, according to the book "Mary Ellen's Guide to Good Enough Housekeeping," by Mary Ellen Pinkham and Dale Burg (2002).
You can remove the blue copper stain from your toilet bowl by combining a cup of ammonia and a cup of water, then scrubbing the stained area. Ventilate your bathroom and wear gloves while cleaning. After the stain is gone, rinse the area and flush the toilet, recommends the book "Cutting Expenses," a guide to home care by Anne Hart (2005).
If you have acidic water or naturally high levels of copper in your water, the blue stain will return, because the copper will build up all over again. Cleaning your toilet bowl regularly will stop the blue stain from becoming large or too difficult to remove.