Ever since primitive man an attraction has existed toward human blood. It is thought this attraction originated in early man, when our observant ancestors noticed that a man losing blood would eventually weaken and die if the bleeding did not stop. It was concluded that the blood contained the man's life. (Have a nobel prize, whoever figured that one out. Also note Xander's comment in Buffy vs. Dracula: "Blood is life", which is repeated by Spike in The Gift, and is also a quote from the book Dracula.)
Gradually over time, we decided to give a magical connection to blood too. Not only was a man's blood supposed to contain his life, but also his experiences, characteristics and qualities. Soon it was believed that these characteristics and qualities could be passed onto others if they consumed or touched the blood. Apparently this works due to some magical Law of Contagion.
In ancient Roman it was thought epilepsy could be cured by drinking a slain (or is it slayed?) gladiator's blood. It was believed that the gladiator's strength and healthy vitality would be transferred to the epileptic. The blood had to be consumed immediately, however, before these qualities dissipated. Pliny wrote in his "Natural History" that some epileptics found it more effective to vigorously gulp a gladiator's blood as it spurted from his wounds, before he died. (euggghh)
The Hungarian Countess, and sadist, Elizabeth Bathory bathed in human blood to preserve her looks (she obviously thought she was worth it). The blood supply came from peasant girls who she kept chained and locked in her castle cellar. Their youthful blood was supposed to keep the Countess' skin looking young. When arrested in 1610 the bodies of some fifty girls were found in the cellar.
The blood of an executed criminal also contains a strong attraction for some people. It is supposed to be a strong protection against disease and misfortune. The blood is said to carry the person's vigorous energy, since he died perfectly healthy. Spectators at the executions of Charles I of England and Louis XVI of France struggled to dip pieces of cloth and handkerchiefs in their blood to preserve it.
When F.B.I. agents shot down John Dillinger, the notorious killer and bank robber, on a Chicago street in 1934 people around quickly dipped handkerchiefs and pieces of paper in his pool of blood. Women even got it on the hems of their skirts and dresses. The source of blood was soon gone but enterprising merchants soon fabricated more and sold it in large quantities.
To some this may seem absurd, but the act continues daily in the consecration of the Catholic Mass. This ritual comes all the way from the Last Supper recorded in the New Testament of the Bible (Mt. 26:26), and I'm sure you must all be fairly familiar with it even if you're not Christian. The priest mystically changes bread and wine into the body and blood of Christ and consumes it in a ritual sacrifice (sadly without pie). Other parishioners then follow and consume it too, for the purpose of receiving Christ and His blessings into themselves. Protestants also remember this act in their communion services.