A pomme de sang, or "apple of blood", refers to a person who has agreed to feed a vampire regularly. In vampire society, the pomme de sang is a submissive but valued position, similar to a kept man or woman among humans. A pomme de sang is protected from interference by other vampires, and is typically given gifts and otherwise treated as a human might treat a lover or, especially in the case of older vampires, a pet. Sex is a completely optional part of the relationship and has to be initiated by the pomme de sang, and is only rewarded to them if they have been good.
In traditional vampire banquets, like those held by Belle Morte, a pomme de sang would eat from the floor instead of being given a seat at the table. Some vampires who are even older than her wouldn't even allow a pomme de sang to sit on the floor. However, regardless of this demeaning treatment, feeding from a pomme de sang makes that vampire duty bound to protect them and to reward them for their services. Money as a reward would be insulting.
Likewise according to vampire tradition, while it's frowned upon to ask to feed on someone else's pomme de sang or human servant, it can be a great courtesy and honor to offer to share them. In Cerulean Sins this tradition is twisted when Musette brings a fifteen-year-old pomme de sang and offers to share her with Jean-Claude if he shares Jason with Musette in turn. Jean-Claude has to refuse since it's illegal to feed from an underage person, and he suspects this move was intentional and designed to test their reactions.
In Danse Macabre, Elinore proposes the idea that they request pomme de sang candidates for Anita as a means to keep the visiting Master Vampires from causing problems while in town. The master vampire Samuel offers his three sons, Sampson, Thomas, and Cristos, since Anita's ardeur might bring them into their power as sirens. The master vampire Augustine offers two werelions, Haven and Pierce. Anita ends up not meeting the other candidates. Sampson stays in town as a candidate, but Jean-Claude later sends him home when The Harlequin arrive.