In addition to Finwe's answer, I believe the disfigured moon may also have been the emblem of the Witch-King himself, though his armor and shield are not blazoned, to my knowledge at least. Obviously, the moon disfigured represents the disfiguring of the moon of Minas Ithil, the Tower of the Moon...but that was aforementioned.
Actually, just for extra, here is a site with some renderings of Middle-Earth's many emblems and flags for the viewing.
Here
Actually, I just realized, that's not entirely accurate, but it's still interesting to a certain extent. Hope that answers your question fully.