The film was based on Russell Thorndike's "Doctor Syn - A Tale of Romney Marsh," but Dr. Syn's name was changed to Blyss to avoid legal problems with Walt Disney, who claimed exclusive rights to Thorndike's stories.
Hammer was originally due to film "I Am Legend" (the Richard Matheson novel) under the title "Night Creatures", but this was abandoned when the BBFC informed them that they would not pass the film. As Hammer had promised the U.S distributors a film called "Night Creatures" the title was passed on to the already completed Night Creatures (1962) instead.
When Dr. Blyss enters the church with Harry towards the end of the movie, he throws his mask onto the ground, yet almost immediately when Mr. Mipps enters the church with Imogene, the mask isn't there anymore.
Why is the main character Reverend Blyss, and not Dr Syn?
At the time, Disney was planning a film based on the same story. To avoid legal disputes, Hammer changed the name of the central character to Reverend Blyss, rather than Dr Syn, as in the original novel.