I get a lot of black 'gunk' collecting on the taps and shower head,
places in the shower cabinet where water collects, etc. I always
assumed it was something from my cold water tank, but since I moved
house to a pressurised mains water system I can only imagine it's now
coming from the hot water, or the cold main supply. It also collects
inside the toilet cisterns around the inlet valve, and under the toilet
rim.

I've not idea what it is or if it's harmfull, but it's easily removed.