Wikipedia
Changing bag
A changing bag is a photographic bag specifically designed to be light-proof while in use. It is required for certain applications involving photosensitive materials when a darkroom is not available, like in the field. Common usages include removing film from its canister to put it into a developing tank, or loading and unloading sheet film holders. They are also commonly found on the set of a film, where the clapper loader may need one if shooting on location or far away from a darkroom.
Usage examples of "changing bag".
Pleased that he wouldn't have to discard the Pentax after all, he put it back in the biscuit tin and turned his attention to the changing bag, giving it a little shake.