The blue disinfectant some people use in their toilets can cause discoloration of your tap water if your water supply was recently turned off. A condition may have been created in which the water from the toilet tank was siphoned into the plumbing of your house. This can happen when the toilet is upstairs and the water supply has been shut off for some reason. 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. Back to Troubleshoot Water at Your Tap
Blue (or blue-green) water may be due to extreme copper plumbing corrosion. 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). If the blue color is only in the hot water, it may be due to the temperature on the water heater being set too high. If you have a hot water circulating system, the return line may be too small or the water may be pumped too fast for your pipe size, or it may be installed incorrectly.