Turning black may mean it is damp. Any holes from the bees to above the crownboard? Many leave feed holes open, which is, frankly, a stupid idea. Is the hive roof leaking? Is the roof space ventilated, so that any moisture, which could accumulate in that space, can be cleared away?
As Swarm says, clean it up. Dryness is not conducive to mould. Insulate between that closed crownboard and the roof to make the crownboard warmer and all will be as well as can be in that area.