Methylene blue is a real chemical compound, discovered in 1896 (by Heinrich Caro), which does indeed have the ability to counteract cyanide poisoning. This property was discovered in 1933 by Dr. Matilda Moldenhauer Brooks of San Francisco.
It will not, however, restore life to those who have died from cyanide poisoning.
The bottle of methylene blue is a real one from the actual firm of Coleman & Bell, Co. of Norwood, Ohio (later Matheson Coleman & Bell) now known as Matheson Tri-Gas, Inc.
The following snipe was stapled and pasted on all of the printed material sent to the exhibitors that booked this film: IMPORTANT! The Motion Picture Association's Advisory Council has urgently requested that there be no mention of specific poisons in publicizing "DECOY." Please eliminate all names of poisons (such as cyanide or methylene blue) from the publicity, exploitation and advertising on this picture.
The movie was largely unavailable for viewing from 1970 to 2000, and since its rediscovery has acquired cult status.
Director Jack Bernhard met his wife, actress Jean Gillie, the femme fatale central to the picture's story, in England, where he was stationed during WWII. He intended this film as a vehicle to showcase her to American audiences, but they divorced a short while later, and she returned to England, never to appear on film again. While filming this movie, she also worked on L'affaire Macomber (1947) (in production April-June 1946). She was more or less forgotten by the time of her early death in 1949 at the age of 33, allegedly a victim of pneumonia. This film marked her U.S. motion picture debut.