[torontophil]

In theory it could work as you have suggested. It seems when sisters take oaths to the Dark One it breaks all previous oaths, enableing them to lie. Late in the series (i wont get into specifics in case some havent got that far) they prove that if a black sister retakes oaths on the oath rod they are bound by them as if they were not black ajah.

So yeah, the Amyrlin could technically do what you suggest, but you have to think about the political implications of doing so. a) you would be admitting publicly it exists b) even if the outside world never found out it would still cause huge schisms in the tower and c) if the outside world did find out the black ajah existed then they would trust aes sedai even less. Plus a whole host of other unforseen consequences.

Cheers,