Wait, there's a secret Ajah?
The Aes Sedai may wield great power in The Wheel of Time, but they are also constantly in danger. With their abilities and knowledge, the Aes Sedai are naturally in the middle of most conflicts. Seeing Moiraine defend the Two Rivers from the Trolloc army with the One Power shows why everyone wants them there. This society of women is uniquely able to defeat their enemies, and with enough Aes Sedai, nothing can stand in their way.
As the Aes Sedai of the White Tower use their power to protect the world, the Black Ajah work against their supposed sisters. When raised to be an Aes Sedai, every woman takes the Three Oaths that prevent them from lying, making weapons, or using the One Power as a weapon against humans, but in joining the Black Ajah, these women renounce their oaths, meaning they are capable of anything.
No other member of the Black Ajah has made herself known yet, but there's no way to tell who may have sided with the Dark One, especially since the Black Ajah has the ability to break the Three Oaths that the Aes Sedai are bound to. The opening scene of Season 2 shows a meeting between Ishamael and the Darkfriends, and although their faces are concealed, one wears an Aes Sedai serpent ring. Of course, this could be Liandrin, but it's impossible to know without more information.