A
dark cloth is really nothing more than an especially nice
blanket. But it allows for the magic to happen...
Sometimes referred to as a focusing cloth, the dark cloth is placed over the photographer's head and covers the back of a view camera at the same time. This is necessary because the ground glass of a view camera only shows a very dim image. Any ambient light stiking the ground glass from behind typically makes it impossible to use the view camera - you simply can't see what you're doing.
Dark cloths also project a certain aura of mystique - hiding the photographer from view while making him or her stick out like a sore thumb in the crowd. Nothing protects identity while projecting presence in quite the same way.
...If you're into that sort of thing
Even better: I know at least one photographer who has specially rigged his dark cloth to attach to the lapels of his photo vest - which has the dual benefit of always have the cloth on hand, and always having a batty, wingy, cloaky thing. Oh what some people do to give their lives meaning...