The plot is fairly common for the horror genre, but in Bava's hands it feels fresh and exciting. Part of the reason for this is the presence of Italian scream queen, Barbara Steele who plays the dual roles of Princess Asa (devil worshipper) and Katia her kin several generations removed. Barbara's big, expressive eyes are perfect for the role and the audience knows she's a force to be reckoned with even before she utters a word. The rest of the cast is very good as well.
Rather than bore you with plot details, I recommend you watch it without any preconceived notions of what's going on. It's part of what makes this film enjoyable. Black Friday is also helped by the make up talents of Eugenio Bava and Mario himself, both of which are uncredited. The Devil mask is perfect and the undead effects are fantastic because they are given the same meticulous detail as every other element in this film. The sets and props are also wonderfully constructed as well.
So, what are you waiting for, SEE THIS MOVIE. It stands among of finest in Bava's career, the finest in horror films, and the finest of all films as far as I'm concerned.
For more info check out the film's entry in IMDB.