Count Chocula (also called Count Chorcula by Ebony) is a breakfast cereal distributed by General Mills. It is Ebony's favorite cereal, and she usually uses blood as a substitute for milk when she eats it.
Seeing as how Count Chocula is not available in England, where the story takes place, it is possible that Ebony regularly has boxes of Count Chocula imported from the United States where it is produced. What's far more likely, however, is that Tara is just an idiot who is trying much too hard to seem gothic by eating Count Chocula because its mascot is a vampire. In real life Count Chocula is not generally taken with blood.