black water in toilet

It did flush & I did call a plumber who came the next a.m. and told me it was not sewage but sediment from the water pipe, probably from a flushing of the community drain, either planned or following a water main break somewhere locally. Problem is gone, whatever it was. Thank you for your response.

If the water sits in the bathtub for a while it turns a blachish color. This doe not happen anywhere else in the house.