I agree that black humor is ultimately used in the end of "Lamb to the Slaughter"; however, I also believe that it is a perfect example of dramatic irony, as well, because the audience is aware of what is going on as the officers eat the lamb.  The characters are not aware, however, that they are eating the murder weapon.  Just a thought.

Black humor is the literary device used in Lamb to the Slaughter. Black humor is the use of the grotesque, morbid, or absurd for darkly comic purposes.

The ultimate example of black humor in "Lamb to the Slaughter" is at the end of the story when the policemen and detectives are sitting around the Maloney kitchen table, speculating about the murder weapon while they unwittingly devour it.