The first recorded instances of blood libel actually occurred in
pre-Christian times, still leveled against the Jews, but by
the pagan Greeks. In 2nd century A.D. it was leveled against
Christians by pagan Romans. It wasn't until the 12th century
that it was level by Christians against Jews; in 1144 in Norwich,
England, a Christian boy was murdered just before Easter, and the
local Jews were accused (with no evidence) of crucifying him in a
mockery of what happened to Jesus. The legend spread from there,
and in the 20th century has been picked up by Muslim
anti-semites.
Some of the nonsense that went with the blood libel was that
Christian blood was needed for Jewish men to replenish their blood
supply, since they menstruated, and also that they needed to make
up for the blood the lost from circumcision.
The blood libel accusation has also been made by some
protestants against Roman Catholics in the 19th century, and
against protestant missionaries by the Bulgarian Orthodox Church.
Primary source: http://www.religioustolerance.org/jud_blib.htm