The black mould will be in parts of the machine that you cannot see so easily - take the powder dispenser drawer out and look at the roof of the slot, for example.
Once a month run the machine empty, on it's highest setting with the cheapest biological powder you can find. It must be the powder, and must not be colour kind or anything like that.
That will kill the mould in your machine.
It builds up because we all wash at lower temperatures, using non-biological, non-bleach detergents which doesn't kill the bacteria that thrives in these warm, moist places.