At the Battle of Shadar Logoth at the end of "Winter's Heart", all of the remaining Forsaken show up to stop Rand's use of the Choedan Kal. The circle being led by Verin finds itself facing one described thusly:
Verin frowned at the apparition making its way through the trees off to her left. She could think of no other term for a woman walking through the forest in gems and a gown that shifted through every color from black to white and sometimes even turned transparent!
During the fight, however, Verin cannot see the woman's weaves:
Using the full strength of her circle, she wove her shield, and watched aghast as it rebounded. The woman was already embracing saidar, though no light shone around her, and she was immensely strong! Then she had no time for thought of anything as the golden-haired woman spun around and began channeling. Verin could not see the weaves, but she knew when she was fighting off an attack on her life, and she had come too far to die here.
The only female Forsaken whose weaves would be invisible to Verin is Aran'gar, but the only one who matches this description (blonde hair, streith gown) is Graendal. Aran'gar has black hair, and the only other Forsaken to wear a streith gown - Moghedien - also has black hair.
Individual weaves can be inverted so that they're invisible to other channelers, but I'm not aware of any case where a woman hides the glow of saidar. There is a way to hide the ability to channel, but not when actually embracing the Source, right?
So why couldn't Verin see the glow of saidar or the weaves being used against her?