The Black Sleep has a stellar cast including Basil Rathbone, Lon Chaney Jr., Bela Lugosi and John Carradine. However, this film is Rathbone's show. He is front and center in nearly every scene as "mad scientist" Dr. Cadman who experiments on people's brains in order to discover a way to cure his wife's brain tumor. Rathbone is his usual excellent self and delivers every line with British conviction and precision.
Disappointingly, Chaney and Lugosi only appear in small cameo roles that are less than representative of their remarkable careers. Lugosi, in particular, looks old and tired which is the result of many years of struggling with drug addiction. Carradine has a slightly bigger role as Borg. His character is fun and crazy and leaves you wishing they had used him throughout the film instead of saving him until the end. As an added bonus [if you're really paying attention] you'll catch a glimpse of Tor Johnson, who was made infamous in the films of Ed Wood. Nice!
The Black Sleep is not a bad film. It's just not all that interesting. The direction and cinematography are solid but the plot is simply lacking something. It's predictable and formulaic and brings nothing new to the table. If you're a Rathbone fan, then you should definitely watch this one. If you're a Lugosi or Chaney fan, you can skip this one altogether.
For more info check out the film's entry in IMDB.