Dark Shirt is an object unobtainable by players through normal means. This item's information is present in the game files, but not intended by developers to be in players' possession.
Dark Mask is the helmet piece of the Dark Set, a costume armor set. This is not to be confused with the Dark Disguise Shirt which is similar, but has a slightly lighter appearance and is not pure black. It emits a faint shadow of smoke when worn.