The black stuff, often slimy, growing in a toilet bowl is unsightly and sometimes difficult to eliminate entirely. It could be caused by more than one thing.
The black stuff is likely black fungus or mould that grows in moist conditions, such as a toilet. The accumulation of substances found in human faeces aids its growth in the toilet bowl, as chlorine dissipates from standing water in a toilet bowl. The high level of humidity in a bathroom also contributes to mould growth. Thoroughly and routinely cleaning the toilet with a toilet brush and bowl cleaner, especially the area beneath the bowl rim, should keep the black mould at bay.
Deteriorating Tank Components
Another possible cause of the black stuff in a toilet is from rubber tank components that deteriorate over time or because of the effects of in-tank cleaners. A deteriorating flapper, most notably, is often the cause. Deteriorating washers on the tank-to-bowl bolts inside the tank can also occur. With gloves on, replace the flapper or other components inside the tank that may be deteriorating. Also, don't use in-tank cleaners.
The black mould won't just be growing inside the bowl, so be sure and clean the area between the tank and bowl and seat hinges, as well as other tight spaces on the fixture. Use a borate-based washing powder to clean the toilet surface.