Plasma

Plasma is the Sim-equivalent of blood. It is used by Vampires in The Sims 3: Late Night and The Sims 4: Vampires to satisfy their thirst motive, which they have instead of hunger.

The Sims 3: Late Night
As stated in a Twitter session with Grant Rodiek, vampire Sims are also able to grow plasma fruits, which they can buy from the grocery store, at night in their own personal gardens. Vampires will also be able to eat plasma packs, which function the same as snacks for human Sims. A vampire can also raid the hospital for Plasma packs, or raid the grocery store for Plasma fruits.

If Vampire Sims have the Vegetarian trait, they are limited to only plasma fruit, instead of plasma from other Sims. If a regular Sim ingests any plasma product, they will become ill. Plasma packs are only available to Vampires, and regular Sims can't drink from Plasma packs.

The Sims 4: Vampires
Plasma and Plasma Fruits return in The Sims 4: Vampires game pack. You have to ask or compell for a drink on normal sims.

Trivia

 * Unlike in The Sims 3: Late Night, Plasma and Plasma Fruits in The Sims 4: Vampires are purple in color instead of red.
 * Plasma Fruit plants are bushes in The Sims 3: Late Night, and trees in The Sims 4: Vampires.
 * It is presumed that Plasma is used instead of Blood in order to maintain a low age-rating for the respective games.