Welcome to the fascinating, but often annoying, world of micro-organisms! If you are experiencing dark gray/black stains or slippery residues in the shower, in a toilet, at a tap, or in the washing machine, you do not have a water quality problem, and you need not fear a health issue. These residues indicate the presence of naturally-occurring mold (fungi), possibly in combination with bacteria, which are commonly seen in our area, and are generally harmless.
Just as is discussed for the bacteria that cause the pink stains, mold spores that result in the slimy black residues are present in leaves, soil and mulch, especially when moist. They enter your house through open windows and doors, on your pet, and on your own hair and clothing; you truly will be unable to keep them entirely out of your home. As with the Serratia bacteria, the spores that find a moist environment, especially where it is also dark and air flow is limited, will be more likely to survive and thrive. In addition to the locations listed above, the drain tube and bucket of a dehumidifier, and the water dispenser spout of a refrigerator, are other typical places the black film may be found.
As with Serratia, the mold will not survive in chlorinated drinking water. A constantly damp surface where the water stands long enough to lose its residual chlorine disinfectant, however, will serve as a prime growth site – your shower curtain, along the flush pores of a toilet, or inside the aerator of your kitchen faucet. Wipe the walls and door, or curtain, of the shower, and spray or mist with a product that contains bleach or other disinfectant. Remove and soak your sink aerators in a dilute bleach solution, using an old toothbrush to scrub them. Use a cotton swab soaked in bleach to disinfect the refrigerator water dispenser. (Caution - our water customers who are served by a septic system should always limit the amount of bleach or other disinfectants that enter the drains).
Note that another form of black staining is not microbial in nature, but due to the chemistry of the water. This is the black staining related to excessive manganese in the water, and is quite commonly seen with the untreated water from private wells in our area. Manganese removal will require a specialized water treatment.
Note also that the discussion above does not include the black stains that may be present on the interior side of cinder block, or on drywall, plywood, or other surfaces where a moisture problem exists, such as in a basement, crawl space, or under vinyl flooring . This is mold of other types, and may possibly be a health concern. The RLMA recommends that you call a mold remediation specialist or a certified home inspector if you require assistance in this area.