While the Shaw Brothers are definitely best-known for their martial arts flicks (as evidenced by a couple peeved reviews on this website), they did dip their fingers in other genres here and there. BLACK MAGIC was one of their forays into the horror sub-genre, telling the tale of a black magician and his various customers looking for love, murder or both.
BLACK MAGIC contains some nastiness and some awesome here and there: we've got rice given magical qualities by female genitalia, breast-milking, dead folks dissolving into maggot-infested skeletons and laser beam-shooting skulls. The problem, however, is that in between a few awesome scenes, the movie does really drag and gets quite repetitive (the black magician sets a curse, the good magician reverses the curse, then the black magician reverses it again, etc., etc.). It's all watchable, but it gets pretty run-of-the-mill near the middle. Luckily, the last 5-10 minutes are fantastic and worth wading through the mediocrity.
Overall, this is worth a look for Hong Kong horror fans, but you could do better if you looked around. For example, just a year later, the Shaw Bros. put out a (name-and-theme-only) sequel, BLACK MAGIC 2, which cranks the crazy factor up a couple notches and is basically an improvement in every way.