When you see black stuff of any kind of debris coming out of your sink's drain, you must stop using the sink immediately until you determine the source of the material. Black stuff coming up through any drain in your house may be from a sewage backup, which can lead to serious damage to your home.
When you use your sink, you may inadvertently wash biological materials down the drain, such as hair, skin oils and soap. Some of these biological materials cling to the sides of the drain body as well as the drainpipe, forming a layer of film. This biological film provides a breeding ground for bacteria, which turns the film black. Sometimes the black film comes loose, for instance, when you pour drain cleaners or other harsh substances down the drain.
Whether you have a partial or complete clog in your sink's drain, part of the clog may come loose and make its way up and out of the sink's drain. If your sink has standing water in the drain or if you notice that the water takes longer than usual to drain out of the sink, a clog is likely the source of the black material in the drain. Often clogs are made up of biological material, meaning bacteria breeds in the clogs and turns them black. Using a plunger or a drain auger should clear the clog out and stop more black stuff from coming up the drain.
An obstruction in the vent pipes that connect to your sink's drain line may also lead to waste water coming up through the sink's drain. The vent pipes attach to the branch drainpipes or the pipes that several plumbing fixtures feed into. Without proper ventilation, the waste water from one of the other plumbing fixtures can flow up the sink's drainpipe, eventually coming through the drain opening. You must climb onto the roof to remove the clog with a garden hose and a high-pressure attachment.
The black stuff coming through your sink's drain may be sewage backing up through all of the drainpipes in the house. Normally, backed-up sewage comes through the drains in a basement first since they sit lowest in the house; however, if your sink is in the lowest level of the house, it will be one of the first to have sewage come out of the drain. A sewage backup may result from a problem with the sewer system that the city must fix, a full septic tank you must have emptied professionally or a large clog in the sewer drainpipe between the house and the sewer system, which must be removed by a plumber.