Everyone thought that the taint on Saidin was a "Counter-stroke" from the Dark One, in revenge for the male Aes Sedai re-sealing him in his prison. Not so. The Dark one did what anyone one would have done, if they were being locked up for eternity....he stuck his foot in the prison door. By placing the taint on Saidin, he insured his eventual release. All he had to do was make sure at least one of the male forsaken, bound to him, remained untrapped. This turned out to be Ba'alzamon. You see, whenever a male forsaken channels Saidin, the taint rolls off the bonds of the Dark One, and returns to him...THROUGH THE SEALS. You see, the only way to a break heartstone Ter'Angreal is to force it to do something it was not meant to do. The Seals were designed to prevent the Dark One from leaving his prison, they were not designed to prevent him from ENTERING it. By forcing the Ter'Angreal to do something that is similar, and at the same time, completely opposite of the designers' intent, the Ter'Angreal becomes degraded and brittle. The proof of this theory is how fast the seals broke. When Ba'alzamon was the only Forsaken free (and only half free, at that), it took three thousand years for the first seal to break. Yet, once more of the male Forsaken were loose, several more of the seals broke, all in quick succesion. More of the taint being channeled through them, you understand. It is also why Elayne and Nynaeve thought the seal that they had felt "evil". The taint was in it. Simple. Remember that in the first book Ba'alzamon told Rand to his face that that he was a fool to think the Dark One had ever been bound. In truth, he had not. The door was locked, true but he had made himself a key.