The Dark Gift is a term for the vampiric power. When a vampire maker bestows the power to become immortal that is transmitted through the Blood on a fledgling, that maker is offering the Dark Gift.
According to the rules of Santino's coven, the Dark Gift was to be given only to beautiful mortals, as an insult to God. Although most of the vampires in The Vampire Chronicles do indeed select beautiful victims, they are motivated by love rather than any rule. Magnus alone is old and ugly, which is why he had stolen the Dark Gift, for he had never been targeted for immortality.