Could it be a Cladosporium or possibly (but not as likely) a Strachybotrys?

Are you sure it is organic? Our very mineralized well water has both ferrous oxide and ferric oxide in it. The ferric oxide is the usual rust color, but the ferrous oxide, which attaches as little nodules to the sides of the pipe in the well, is black.

I moved into a new house a year ago in South Cheshire and also have the same problem. Black matter forms on the shower head and cold taps and no-one else i have spoken to seems to know what it is.

I have a spa bath, which I rarely use. If I do so, these emerge, so I have to let it run with some Milton to disinfect, empty and refill before using. I have to assume it is something growing in the stagnant water in the pipes. It is somewhat spriggy in texture, and doesn't feel metallic or hard in any way.

This answer has been selected and edited by New Scientist staffI know this black jelly-like material well because it formed an integral part of my PhD studies, which were focused on the sort of water filters mentioned above. The material is a biofilm: a jelly made up of a layer of bacteria and their extracellular products. These commonly form at the interface between a solid and a flowing liquid.The black coloration comes from manganese, which is common in groundwater. When water emerges from a spring, any manganese it contains will be in the form of soluble ions. Part of the treatment process is to oxygenate the water with an air cascade, and in some areas chlorine is added to the water to oxidise both iron and manganese. The insoluble manganese oxides created in this process are filtered out before water enters the distribution system.An alternative to this chemical oxidation is to allow a biofilm of manganese-oxidising bacteria such as Leptothrix species to form within a sand filter as the water passes through the filter. This film can build up into a sheath that, in the case of Leptothrix discophora, can be up to 20 times the diameter of the bacterial cells.The ribbon-like nature of the slime shows that the biofilm has formed under conditions of fluid shear. In effect, the film is stretched out by the flow of water. These ribbons are known to biofilm researchers as "streamers". Your questioner is seeing a biofilm of Leptothrix coloured black by manganese oxide. The water is safe to drink: manganese toxicity is not a problem in municipal water systems, but it can cause staining in laundry and plumbing. These biofilms are not at all dangerous, just messy.Chris Hope, Lecturer in oral biology, University of Liverpool, UK