This is a phenomenon that occurs in areas where hedro is accumulated on the seabed. Hydrogen sulfide emitted from the seabed reacts with cuprous oxide contained in the ship’s bottom paint to form copper sulfide, which causes blackening. Hedro tends to accumulate in places where there is no or little tide in the city where domestic wastewater tends to accumulate, so as you asked, it can be said that it is a phenomenon that often occurs in some areas. If it turns black, the antifouling effect of the ship bottom paint will be lost, so it is best to apply a paint that does not contain cuprous oxide (eg UNAGI Rainbow) to the ship bottom paint. Alternatively, you can wipe it with thinner and repaint it, or if the area is heavily polluted, increase the number of coats.