Door Seals Going Black

Is anybody else having the problem of door seals on all makes going black, I can only guess that it is a form of mould but you cannot remove it as it has stained the seal or possible the seal has gone back to its original colour?
I have been repairing machines for over thirty years and this has only happened in the last five or six years, I hardly ever changed a seal on a Zanussi but even these I have done at two years, I even changed a Hotpoint one that was only 9 months old!
I live in the south east and the water company is Southern Water, is it something that is added to the water?
The soap draws on machines are going black too so it must be in the water.
Does anybody have any answers ?

Bernie