Very good ~ I’d thought maybe it went all the way back to Passover’s origins. Maybe not quite, but there it is.
“The origin of the blood libel can be traced back to the Graeco-Egyptian author Apion, who claimed that Jews sacrificed Greek victims in their temple. Apion repeated anti-Jewish slurs and “absurd calumnies” first made by Posidonius and Apollonius Molon in the 1st century BCE.”
This is the earliest I can find. Probably started with the cavemen.