The concoction of the same metal from the pendant – that pricked and almost killed Blair, and her blood, was the only thing that Maeve put together to create the mixture that could heal Lucien.
Blair’s blood would serve to be the venom, reversed by the power of that pendant from the heritage of vampires, which was yet the best way that Maeve could think of to heal Lucien. Not a perfect assurance, though. Maeve knew that this could be her first try in that case.
It probably was as impossible as punching the moon. However, she had to do it anyway. Lucien was on the verge of dying, and as much as she wanted him dead, she couldn’t gamble the trust Blair had for her.
Her plans for the future were more than just having Lucien dead.