I was thinking more about her role, and wonder if she was turned to the Dark One by the 13 Myrddraal/13 Dreadlord technique. She was the one, after all, that told Egwene about it being possible in the first place. Plus, prior to the Tower coup, Min viewed her, and saw her face battered and bruised, plus we know she was among the leaders of the rebel exodus.

It got me wondering if Mesaana had to beat her to get her to lead the blues and the others from the WT to spread the dissension the Shadow sought. Not only that though, we know that Sheriam also received at least one beating while the rebels are on the march to Tar Valon. It also appeared Mesaana had to use force to get her to comply with stealing the dream ter'angreal stash, and the guards said she was "flustered" when she took them. Plus, at her beheading, Egwene says Sheriam seemed relieved that it was over, even confessing to crimes without needing to be interrogated.

These things kind-of point to her being a reluctant Black, rather than an eager, power-seeker, or, Mesaana just liked using a heavy hand with her. At any rate, it seems too late in the story to do a forced-turning, so maybe we already had one in Sheriam.

Just a thought.