I knew this was wrong because I had just read Bellum Catilinae and, although he mentioned that there was rumor about drinking blood, there was no mention about where the blood came from and he stated that it was a rumor. So, how did Robert Harris get the idea to put a murdered child in his historical novel? Somebody must have written something about that at one time.
Marcus Tullius Cicero, since he got a great deal of political mileage from frightening people about a conspiracy that was plotting the violent overthrow of the government, to burn Rome, and kill hundreds of citizens, if there was any suggestion that there had been anything like taking oaths by drinking blood going on at the meetings of the conspirators, would have been all over it. It would not even have to be true, just a whiff of a hint would have sufficed. But, he said nothing about this in his speeches in the Senate against Catiline or in his letters.
Approximately twenty years later, when Sallust was writing his history, he mentioned the rumor. Plutarch, writing 100 to 150 years later in Greece, wrote in his biography of Cicero that Catiline's men had murdered a man and drank his blood. Dio Cassius, writing 200 years after the events, related in his history, Book 37, section 30, that Catiline killed a boy. The tale grew in the telling, like the game of telephone.