Reading this question about identifying the key, I recalled that in the final episode of season five, The Gift Buffy functioned as the key that closed the portal.
Would her blood be enough to open it?
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Very likely. According to the Buffy Wikia:
Knowing the portal won't close until her blood stops flowing, Dawn plans to jump into the portal, believing that doing so would close it and save the world, even if she dies in the process. Buffy stops her; she then suddenly has a flashback of several points in the season -- telling Dawn they both shared "Summers blood", her earlier proclamation that "the monks made [Dawn] out of me," and most significantly, when she met the spirit guide, who told her that death was her gift. Buffy realizes that she and Dawn share the same blood, and that if she (Buffy) throws herself into the portal, it will close. She also realizes what the First Slayer's message really means: instead of being what she has to offer people, death is actually a gift for Buffy.