About the only analysis that I can even find on that portion is from this website, where it is compared to the Qabalah (or Kabbala) and the Tree of Life. I don't remember there being much of an explanation in the original book, "The Club Dumas".
Earlier, when Corso first expressed his admiration and gratitude to the girl for fighting for him against the albino down by the river, we saw the bond of Higher Feeling growing between them. It was at this point that the girl looked lovingly at Corso and drew with her fingers three vertical lines on his forehead with her own blood, the central line extending down his nose. There is a long spiritual tradition of marking the forehead as a sign of spiritual protection and initiation in both Eastern and Western traditions and three vertical marks on the forehead is the traditional Hindu sign of initiation of those who worship God in the form of Vishnu, the Preserver, whose qualities (like those of Chesed) are kindness and mercy. Such marks are also worn as a reminder of the consecration of the body as the temple of the Spirit, and in Corso's case might represent a sign of his initiation into Higher Feeling and the realm of the Soul. [Corso's blood stained forehead is not immediately cleaned.]
In the context of the Tree of Life, the three symbolic marks might also refer to the three Transcendent Creative energies of the Supernal Triad (which, when the body is superimposed on the Tree, are located in the head). The elongated center mark might symbolize the Central Pillar which extends downward from the highest central sephira of the Godhead, Kether/Crown, representing the route of the path of return which Corso is traveling. The girl begins as a symbolic figure who models for him the qualities of the Soul which he will gradually grow into, but once Corso has achieved the union of his ego and Soul, the inter-dimensional figure of the girl remains a symbol of the non-dual Spirit. It is his union with her in this capacity which will carry him across the Abyss and into union with the Spirit with the opening of the Ninth Gate.