Sounds like it could be an insulation issue. Where the cold is meeting warmth abruptly, it condensates onto the coldest surface. The same as condensation on glass, but in cases like these, condensation forms and black mould builds up.
The answer : slow down the temperature change by insulating either the wall or units. You can use insulated plasterboard or insulate the wall externally, or just by creating an air break between the wall and unit (like a small void or cavity). For a cheap fix some people try thin polystyrene (like wallpaper type) and line the wall units with that, but its not going to be as effective.