Black mould generally grows due to too much moisture.
For hard cheeses that are wrapped you may see development of black mould if the cheese has been washed too frequently.
For mould ripened cheeses (Camembert, brie, blue cheese) it may be due to the surface growth of the mould being so thick that it holds excessive moisture.
To avoid both of these scenarios ensure you sterilise all your equipment extremely thoroughly prior to making cheese. Also make sure your cheese isn't too moist when it is wrapped.