Having blue water is rare. The blue disinfectant you use in your toilet can cause discoloration of your tap water. If your water supply was recently turned off, a condition may have been created in which water from the toilet tank was siphoned into the plumbing of your house. These disinfectants contain chemicals that may pose health hazards if ingested or touched. Flush your plumbing by opening each tap until the water runs clear. Do not drink this water.

Blue (or blue-green) water may be due to extreme copper plumbing corrosion, which is also a rare circumstance. If this is happening, the water will usually have a bluish-green tint and/or will leave a bluish-green stain around fixtures and on a white surface if the water drips from a faucet. This copper corrosion can be caused by your electrical system being grounded to your water pipes, especially if you have a mixture of pipe material (i.e, some copper and some galvanized iron).