Vincent Price was first offered the part of Paul Henderson but matters including his exclusive contract with "American International Pictures" pertaining to his involvement in horror films proved to be an impediment.
On this movie's initial U.K. release, the British censor wanted to give it an A rating (equivalent to PG) because of its lack of gore. Fearing that this would harm box-office returns, the distributors asked for it to be re-rated X or they wouldn't release it. The censor concurred.
Despite the title, not one drop of blood is seen in the entire movie.
Peter Cushing (Philip Grayson) tried to get out of his contract so that he was not away from his sick wife Helen, but he had to carry on. Helen Cushing died in January of 1971, a month or so before this film was released.
Christopher Lee (John Reid) can be seen reading his favorite book, "The Lord of the Rings" by J.R.R. Tolkien, thirty years before he played Saruman in the three movies based upon it: "The Lord of the Rings: The Fellowship of the Ring (2001)," "The Lord of the Rings: The Two Towers (2002)," and "The Lord of the Rings: The Return of the King (2003)." Lee read the book once a year.