Alfred Hitchcock's first sound film utilized the new sound technology in a rather creative way off-camera.  Hitchcock's lead actress, Anny Ondra, had a strong Eastern European accent that was difficult for English audiences to understand, so Hitchcock's solution was to have British actress Joan Barry speak Ondra's lines of dialogue off-camera.

The film concerns a woman who kills a man who tries to assault her.  Ondra plays Alice White who, while having dinner in a fancy English nightspot with her husband-to-be Scotland Yard Detective Frank Webber (John Longden), begins to flirt with an artist (Cyril Richard) seated at the next table.  The artist invites her up to see his studio, and she goes but balks when the artist asks her to pose in the nude.  When the request becomes a demand, Alice stabs him to death.

She rejoins her fiancée and tries to forget the murder, but her conscience keeps bothering her.  To make matters worse, sniveling rat Tracy (Donald Calthrop) materializes to blackmail Alice for the crime.

Blackmail is Alfred Hitchcock's first talkie, and not a bad effort at all.  The whole film was almost completed, when sound came in and revolutionized the industry; Hitchcock was forced to re-shoot some sequences and add others to make the film a mostly-talking film, something like Alan Crosland's The Jazz Singer (1927).

Hitchcock's leading lady, Anny Ondra, had a very thick continental accent, not a problem for a silent film, but a real liability for a talkie.  Hitchcock overcame the problem by having another actress speak the lines onstage, offscreen, while Ondra simply mouthed them for the camera.  Since dubbing was unknown at the time, this was the only method; then, too, the camera was confined to a soundproof shooting booth, and so the mobility of Hitchcock's camera is severely limited.

During one long sequence, Cyril Ritchard as Crewe, the artist, sits down and plays a piano solo seemingly to keep the audience interested, but the film ends with a thrilling chase through the British Museum (mostly accomplished using miniatures, and the Schufftan Process, which allowed full-scale backgrounds to be reelected into the lens of the camera through a series of mirrors).

It's interesting to see how Hitchcock deals with sound, when it was clearly thrust upon him at the last minute, and while not a front-rank Hitchcock, it is still a remarkable historical document of an artist finding his way through a medium that has suddenly been transformed by advancing technology.