Examples of ritual murder of enemies and consumption of their blood (or other organs) are known from many cultures, and continue to be rumoured amongst some subcultures. For example, in 1840, a rumour spread in Syria that a priest, Father Toma, supposedly went to the Jewish Quarter of Damascus and never returned. The story was that the priest was slaughtered by a group of rabbis and other Jews. Not a drop of blood was spilled. It was collected, so the infamous "blood libel" story went, and used to make Passover matzoh. The "blood libel" story has been told over and over again through the years, with other victims, such as Arab children. The story is still told and continues to influence Arab perceptions of Jews.
In John's Gospel, Jesus had participated in three Passovers during his public ministry. John is careful to include these details because of their significance and impact. It seems that in the first two Passovers Jesus prepares his followers for the true Passover where he will offer his own body and blood as the true sacrifice. Jesus Christ is both priest and victim.
People readily confuse the metaphor of drinking blood, notably the "Blood of Christ" during the Christian Mass, with the actual consumption of real blood.