I'm not sure I've got this straight...
You have built cupboards in two of the corners of the bedroom - OK.Yes
They have 'one Air Vent in each' - I assume you mean the WALLS leading to outside have vents in?No on the side of cupboards .
You 'leave the door open' - What door? The cupboard door? The bedroom door? Both
You get 'black mould on the outside walls in cupboard' - I'm lost on that one. Where exactly is the mould? On the actual brick wall? Yes .Inside the cupboard?Yes. On the outside of the cupboard?No.The cupboards are built into the corners of bedroom floor to ceiling.so if u imag a box 2 side are wood and the other 2 sides are outside brick and plaster walls.9 inch no cav.
It sounds like a ventilation problem but we need you to clarify the points above and answer some questions below. Essentially, have you blocked or hindered the ventilation bricks in the walls? If so your problem with the mould will persist until you provide adequate ventilation. Have you 'butted' these cupboards right up to the wall? Have you tried to extend the ventilation by installing a vent in the bottom/side of the cupboards? Do these cupboards go from the floor up, ie like a wardrobe? Opening the window may help but it's cold at the moment so I doubt you'll be wanting to have that open through the night. Which, if you are an average sleeper, is when you and possibly a partner, will spend approximately 8 hours exhaling warm moist air into a room that, if I my assumptions are correct, has inadequate ventilation. The warm moist air you breath out is struggling to get outside. It hits the cool air that is struggling to get inside and so condensation forms around that area, which in turn leads to damp conditions, which in turn can lead to mould.
However, as I have already pointed out above, I may have misunderstood your description and so there may be other factors effecting this.
We need you to be more descriptive. Particularly, if you could address the points/questions above.