If an Aes Sedai becomes Black Ajah, the Warder would know instantly that something was up, but wouldn’t know exactly what. The Black Ajah has three choices, then—hope the Warder is a Darkfriend or amenable to being one, hide the affiliation, or arrange for an accident. Yes, this would be painful for the Aes Sedai, but it might become necessary. The process of becoming Black Ajah is evidently quite painful in its own right and thus probably involves more than just swearing new Oaths on the Rod.
(I submit that this is why the Red Ajah is rife with Black Ajah—they have an easier time actively recruiting from that pool. By the same token, I claim that the Green Ajah is more pure than the others.)
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