The Black Cauldron was based on the Chronicles of Prydain, which I think is still considered one of the best children's/young adult fantasy series ever written -- they're absolutely fantastic. But Disney basically combined the plot of the first two books (with a myriad of changes) into one film and it didn't really work. I know the author (Lloyd Alexander) said he thought the movie was very entertaining as a movie, but that it was basically unrelated to his actual books.
With that said, Disney bought the movie rights to the entire 5 book series a few years ago, so they probably have (or at least had) some plans to give it another shot. I assume they'd throw out the old Black Cauldron animated film and start from scratch, though -- it could be a great live action, five movie epic.