"O let the Nameless Ones behold the girl given to them, who is verily the one born ever nameless. Let them accept her life and the years of her life until her death, which is also theirs. Let them find her acceptable. Let her be eaten!"
Other voices, shrill and harsh as trumpets, replied: "She is eaten! She is eaten!"
The figure in black is most likely Thar, priestess of the Twin Gods. The alternative is some unnamed young priestess chosen for the part.
Amongst the priestesses, and now all in black like them, the child walked....
All the signs indicate that this was a mock sacrifice, part of the ritual where Tenar was "Eaten" and turned into Arha, the One Priestess of the Nameless Ones.
The "slenderer arms" are a literary convention for referring to a woman, and the priestesses are all dressed in black.
Very few of the other priestesses in The Tombs of Atuan are named, and which priestess is the "figure in black" depends on whether the role of the rescuer is for someone of high rank or not.
So this leads to the possibility of the figure of Thar or Kossil, the two important priestesses named in the book. Thar is the one teaching Arha the rites of the Nameless Ones, and if anyone would participate in this rite, it would be her. Plus, in the long run, Thar is kinder to Arha than Kossil.
However, "two tall priestesses" guide Tenar to the throne, and just after the mock execution, the figure in black leaps away and then the two priestesses lead Arha away. These are most likely Thar and Kossil. Also, Thar is rather elderly (she dies during Arha's adolescence), and may not be athletic enogh to play the role.
The other named priestess, Penthe, is the same age as Arha and appears much later, so it is not likely to be her.
This all means it's likely that this figure is just some priestess Ms. LeGuin never named.
