Like I Was a Teenage Frankenstein (rushed into production to capitalize on the success of I Was a Teenage Werewolf), Blackenstein was created solely to ride the coattails of a similar low-budget exploitation gimmick, the previous year's Blacula. But whereas Blacula had a certain scrappy charm that has turned it into a cult classic over time, Blackenstein is just an amateurish mess, failing to capture anything entertaining or interesting about the Frankenstein mythos or about the blaxploitation style it's trying to co-opt. It stumbles around as clumsily as its poorly conceived version of the monster, with shapeless scenes, awful prosthetic effects, slack pacing and stilted acting.
Although it's helpfully subtitled The Black Frankenstein, Blackenstein actually features a white guy as its version of Victor Frankenstein, John Hart as Dr. Stein. Holed up in his palatial Southern California estate, Dr. Stein does research on reattaching limbs, and his latest project is Eddie Turner (Joe De Sue), a military veteran who lost both arms and both legs in Vietnam. Stein takes on Eddie's case thanks to Eddie's fiancee Winifred Walker (Ivory Stone), Stein's former protege, who moves in with her mentor while helping Eddie recover. The process that Stein uses makes no real sense ("Dr. Stein just won the Nobel Peace Prize for solving the DNA genetic code," Winifred helpfully explains to Eddie), and unlike Frankenstein, he's not reanimating corpses, merely attaching new limbs to living people (and it's never explained where the limbs come from).
Eddie becomes a monster not as a result of this process, but thanks to sabotage from Stein's assistant Malcolm (Roosevelt Jackson), who's jealous of Eddie and wants Winifred for himself. He switches out some formula in Eddie's injections, and Eddie transforms into a hulking brute who sort of resembles the Universal version of the Frankenstein monster. Eddie wanders off at night and kills mostly random people, eating their entrails like he's a zombie. He goes from a quiet but articulate man in pain to a grunting monster, pretty much the opposite of how the monster develops in Mary Shelley's novel and the better adaptations. There are long, dull sequences of Eddie wandering around deserted alleys, and there's a random interlude in a nightclub, as a comedian tells a rambling joke about a talking dog, and then a lounge singer performs nearly an entire song. Even the climax feels like blatant filler, as Eddie's final confrontation is not with Winifred or Dr. Stein (who take a ridiculously long time to notice that he keeps escaping to kill people) but with a random woman who's never been seen before. He slowly (very slowly) stalks her before being killed by a pack of dogs, even though point-blank bullets had no effect on him just a few scenes earlier.
The best part of the movie comes early on, when Eddie is still in the VA hospital and an angry nurse berates him for buying into the false patriotism of joining the army, and refuses to help him get a drink for his parched throat. It's a glimpse at the blunt social commentary an exploitation movie like this can throw in along with its gore and nudity, but director William A. Levey and screenwriter/producer Frank R. Saletri never follow up on it (the nurse is Eddie's first victim, but he doesn't get a chance to talk again). Instead they just cobble together bits of what has worked for other movies, without any regard for whether they fit together or make sense in context. No wonder the result is such an ugly beast.