The Witching Cloak is a magical item, originally owned by Mister Dark. It later came into the possession of Fabletown. Boy Blue used it in the Homelands. A thread from the Witching Cloak got inside Blue which eventually caused him to die.
The Witching Cloak is supposed to be all unnamed magical cloaks. Willingham's reason for the different cloaks having different abilities is that the owners might not necessarily explore all the cloak's abilities, just use the one ability they first discover, assuming that it is all the cloak can do (while Blue had lots of time to discover many of the cloak's abilities).