Why could Verin not see this channeler's weaves?

Question

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?

Answer 1

The woman was most likely using the True Power

As per the Theoryland results for 'True Power' Robert Jordan explains that you cannot tell if someone is wielding the True Power:

AARONB20
Can you clarify who can tell when one is using the True Power?

ROBERT JORDAN
No one can tell if you're using the True Power except the Dark One, of course.

Not even other people who can channel the True Power can see if someone is wielding it¹:

MONKEYMAN
Nobody who can channel saidin or saidar can sense the True Power. And, since the Forsaken don't seem to notice Moridin channeling the True Power, is it detectable at all (besides by a gholam)? That is, can someone already holding the True Power sense another using it?

ROBERT JORDAN
No. Not by any method we've seen yet.

The passage:

...she wove her shield, and watched aghast as it rebounded. The woman was already embracing saidar...

To me this does not suggest that Verin knows the woman is embracing saidar, instead she is assuming this is the case. We (along with Verin and most all of the Aes Sedai) know that is is incredibly hard to shield someone who is already embracing either saidar or saidin.

and she was immensely strong!

This also explain that perhaps the True Power was being used, as the True Power is stronger than saidar or saidin:

The Great Lord of the Dark provides his most favored minions with the True Power that comes directly from him. It is even more addictive and dangerous than the One Power.
_{WoT Encyclopedia - A Crown of Swords, Chapter 20}

And so, during the battle, the woman is (most likely) embracing the True Power, meaning Verin cannot see the weaves.

_{¹The only caveat I have found is that the gholam can sense the True Power as an itch.}

Answer 2

Graendal had hidden her ability, and inverted her weaves.

We see Leane do this in Knife of Dreams, something any forsaken should be able to do.

“There was something strange, though. More than strange. We couldn’t find the wilder, at first. We couldn’t feel her channeling. There was no glow around her, and we couldn’t see her weaves. The chain just started turning white. If Arebis’s Warder hadn’t spotted the boat, she might have finished and gotten away.”...

“Clever Leane,” Egwene murmured. For an instant, she squeezed her eyes shut. Leane had prepared everything in advance, before coming in sight of the harbor, all inverted and her ability masked. If she herself had been as clever, she likely would have escaped cleanly. But then, hindsight always saw farthest. - Knife of dreams

a second mention in knife of dreams,

"One of them is channeling," Nynaeve said, just loudly enough for him to hear, as she climbed down from her saddle. "I can't see anything, so she's masked her ability and inverted the weave – and I wonder how the Seanchan learned that! – but she's channeling."

And from another passage during the Cleansing of Saidin we see that Cadsuane was also using inverted weaves.

and struck instead the inverted shield she had woven, erupting with a deafening roar fifty feet above her head. Even within the shield the air crackled, and her hair stirred and lifted. - Winters Heart

Answer 3

I don't think they ever said it explicitly, but we know from later books that Graendal had been gifted with some amount of the True Power. If she chose to use the True Power in that attack, Verin wouldn't have seen it either.

That said, if she knew she was embracing saidar, the other possible answer is that she was using inverted weaves,as Egwene and other advanced channelers used at various points in the series.

Inverted weaves are also invisible.

I'll have to look up pages and quotes when I get home.