Ok, this theory is very simple, based on a slight play on words. In CoT, when they are in So Harbor, Perrin is in the warehouse, hears a scream, and comes out. Senoid is there, and says to him "I find it hard to lie." Now to see how this is important, I have to examine the Three Oaths, well 1 of the oaths.

1)To speak no word that is not true.

This should put a rest to most argument about what this oath entails. As long as the words are true, any actions, voice type, or anything else can implay anything you want. This means, that the words have to be exactly true, and that Aes Sedai can't say something false and concider it as truth because they are being sarcastic. Now analyze exactly what she says, "I find it HARD to lie." That in itself has to be a lie, because it seems impossible to lie with the oaths in place. Some people would argue that they can lie if they believe the purpose is valid enough, but if that was true then all Aes Sedai would have to do is convince themselves that the lie benifits them enough, and just like that, that oath may as well not exist and Aes Sedai could lie with only a few moments hesitation. This means that by saying it is hard to lie, she is lieing, and that means, that either she somehow took the first oath off, and happens not to be caught, or she is Black Ajah. We have no reason to believe that she took only the first oath off, and this is something Jordan would have somehow forshadowed, and that proves to me that Senoid is Black Ajah.