Dr. Pride has invented a formula that is supposed to, uh. I'm not sure what's it's supposed to do. What it actually does is make whatever he tests it on pale, hyper-aggressive, and endowed with super-strength. Pride tests it on an old lady who comes through his hospital who has no family, no friends, no one to sue him if his experiment turns out badly. (Listen closely and you'll hear even the script itself forget the poor lady's name.) Well, the formula turns her pale, hyper-aggressive, and endowed with super-strength. Darn, who could have foreseen that? The old lady dealt with, Pride decides that testing it on anonymous old people is too dangerous. Obviously, he'll have to test it on himself! To his great surprise, he turns pale, hyper-aggressive, and endowed with super-strength. He goes to town and kills some people who p**s him off.
Now we get to the most bizarre plot point of the entire movie, which is saying something. Everyone who saw Pride/Hyde/whatever during his rampage assumes that he was a very large white man. Pride is played by Bernie Casey, who's obviously pretty black, and the makeup seems nothing more than some flour patted onto his face and in his hair. There is no way in hell anyone would see him and think that he was anything but a light-skinned black dude. Perhaps the script intended Hyde to be played by a white actor. Maybe the makeup looked more convincing in person than on film. Maybe they just didn't care. Who knows?
Long story short, Pride comes to realize he enjoyed being a "white" man, as being white allowed him to commit crimes (mostly against other blacks) without fear of repercussion. I'm sure that was supposed to really mean something but the movie is too indifferently made for it to stick. Anyway, quaffing more Campbell's Instant Caucasian turns into Pride's regular thing, as it were. At the end, Pride/Hyde climbs a tall electrical tower and is shot down by police, almost as if the makers of the movie got bored with the source material and decided on the spot to rip off King Kong.
Dr. Black and Mr. Hyde is not so much a bad movie as a lazy one, which may be worse. Everyone, except Bernie Casey and maybe the chick playing the hooker he befriends, is just going through the motions. Blacula and its sequel made a lot of money, and even that awful Blackenstein movie made its money back, so surely a "black" version of Jekyll and Hyde would turn a profit, right? Bernie Casey delivers a truly superheroic effort in this movie. It's almost like he's performing in a much, much better film that we don't get to see. Dr. Black and Mr. Hyde is almost worth seeing for him alone, plus the old lady's freak out once Pride injects her with his formula is a hoot.