Broderick Crawford's line "He thinks he's Sherlock Holmes" is a gag. At the time this was made, Basil Rathbone had already played in two Holmes films, "The Hound of the Baskervilles" and "The Adventures of Sherlock Holmes."
Director Orson Welles saw "The Black Cat" and was so impressed by cinematographer Stanley Cortez's atmospheric lighting and angling that he hired Cortez to photograph The Magnificent Ambersons (1942) which is also set largely inside a Victorian-era mansion.
That is a human, not a real feline, doing the "meows" looped into the soundtrack.
Universal ordered Alan Ladd's billing increased to take advantage of his notoriety in the current Paramount hit This Gun for Hire (1942).
Marlene Dietrich's from-the-back cameo in the film is due to her stopping by on a break from filming The Flame of New Orleans (1941) to visit her then-boyfriend, Broderick Crawford. She graciously volunteered to fill in for Claire Dodd in a non-dialog scene after the actress left the studio for the day.