"It appears that a considerable number of Ashkenazi Jews traditionally regarded Christian blood as having powerful magical properties and considered it a very valuable component of certain important ritual observances at particular religious holidays. Obviously, obtaining such blood in large amounts was fraught with considerable risk, which greatly enhanced its monetary value, and the trade in the vials of that precious commodity seems to have been widely practiced"

https://www.unz.com/runz/american-pravda-oddities-of-the-jewish-religion/