It's garnered a cult following over the years, but when it released, it had some nasty reviews.
It didn't help that it a terrible development cycle
It should be worth noting that Disney wasn't doing financially well at the time, and it was that way ever since the passing of Walt in 1966, and the studio wasn't sure what direction they wanted to take with their films. This is known as the Disney Dark Age (Or sometimes referred to as the Disney Bronze Age), spanning from 1970 - 1989. The films in this period weren't as financially successful, and usually don't have the magic touch that the older Disney films had.
Anyways, as for Black Cauldron, the film was in development for over a decade, and It ended up becoming the first Disney Animated Film to receive a PG rating. Funnily enough, it was originally gonna be much more edgy in content, So much so, that if they didn't cut the fifteen minutes of scenes that they did, it probably would have received a PG-13 or even an R rating. It didn't help that the film was the most expensive animated film at the time of it's release...
When it was released, it received a pretty negative critical reception, and only made $21 million domestically on a $44 million budget. And to make it even worse, it was out grossed at the box office by The Care Bears Movie which came out around the same time
Disney almost went bankrupt after the film's release, but thankfully, their next movie, The Great Mouse Detective was a critical and financial successes that ultimately recouped it's budget, and gave Disney enough to work on a little movie you may know as the Little Mermaid, which started the Disney Renaissance, a period from 1989 to 1999 where Disney was at the top of their game, releasing critical and financial hits such as the aforementioned Little Mermaid, Beauty and the Beast, Aladdin, and the Lion King...
So let's just say within 5 years after it's release, Disney was doing fine and dandy. Though with all of the trouble it caused, Disney almost never acknowledges the film, and has only been released on home video once in 1998, and Elionwy is not considered to be a Disney Princess
As you may or may not tell, I am a pretty big Disney nut