When Siuan came to tell Egwene that the battle had begun, she described some of the Black Ajah Sisters as being able to channel very strongly. Though Siuan's reduced strength might be a factor, after having been used to Egwene, Nynaeve, Elayne and some of the newer initiate's strengths, Siuan would not have reported it in this way if she felt that they were stronger than she expected. I started to go through the list of known Black Ajah to see which ones would be strong enough to merit a comment, but there were too many unknown Sisters that were members of the B.A. for me to get that far.