Thank you for the question
I saw it once several years ago, but prior to that I had been familiar with the story of the movie because of an audioplay of the movie which I had since about 1991 and which I had listened to frequently.
It is a funny coincident that you happen to ask this just now because a few days ago I listened to some audioplays I had borrowed from a local library. They were the chronicles of Prydain on five CDs (based on the books rather than the movie).
As for a DvD release of the movie, I haven't heard of it so far, but I hadn't investigated about it specifically.
Now this is not a replacement for a DvD of course, but I think that
The Black Cauldron can be found on Youtube too if you haven't seen it so far.