I don't know if this helps at all, but we used to suffer from black mould in the ceiling corners and down the walls. We used the Dettox mildew and mould remover to wash it all down and then we installed an extractor fan with a humidity sensor, this now comes on automatically and stays on for a good while after showering and although it is early days since having refitted the bathroom, there doesn't appear to be any sign of mould. We also previously had a painted wall surface and even though it was kitchen/bathroom paint it didn't help, we have now fully tiled the bathroom.