It's easy to slam
The Black Hole for its bad science, but IIRC nobody really knew how black holes worked at the time. They had only just been discovered when that movie was made. Most of what we now know about them wouldn't be found out until much later.
The thing in the movie isn't a black hole, it's a
wormhole. Back then, they thought those were both the same. In fact if you change the name of the movie to something like
Wormhole, the science doesn't really seem so bad, does it?

(as far as I know, an actual wormhole hasn't ever been discovered, so we can think whatever we want about them)
As for the movie itself, I loved it. Great music. Awesome visuals. A character played by "Maximilian Schell" ending up trapped...in Maximilian's shell. (Don't even TRY to tell me that was a coincidence.) And the thing with the humanoids being the lobotomized
Cygnus crewmembers, creeped me way the hell out when I was a kid.
Probably the only movie where we actually get to see the bad guy end up in hell. I mean, how does a movie with an ending like that, get made by fucking DISNEY?
Actually the only real problem I have with the plot is, how easily Dr. Reinhardt overpowered the other members of the
Cygnus crew in the first place. There had to have been hundreds of them, and only one of him. So how did he get the drop on all of them? That bugs me worse than the "science". I mean, sure, there was Maximilian and the other police robots, but surely the crew could have defended themselves?