“Dark Blood” is a euphemism for the act of being turned into a vampire. Marius often uses the term; for instance, he explains that he became a vampire when his maker, the God of the Grove (Teskhamen), gave him the Dark Blood. Marius also explains that he does not want to condemn Armand to the Dark Blood by turning Armand into a creature like himself.
The idiom appears in The Vampire Lestat (1985), The Tale of the Body Thief (1992), The Vampire Armand (1998), Blood and Gold (2001), Blood Canticle (2003), Prince Lestat (2014), and Prince Lestat and the Realms of Atlantis (2016). For more perspectives on the Dark Blood in the Vampire Chronicles, read the Alphabettery entries Armand, God of the Grove, Marius, and Teskhamen.