I remember watching this movie in the United States in the mid-90s in English but what I remember of the animation makes me think it might have been dubbed from Japanese. I remember renting the VHS and watching it at home, and I think I remember it having a full story arc so I don’t think it was an episode of a television series.
The main character was some kind of musician or conductor. At some point he is in a normal/”real” world, and at some point he is in some strange/trippy world. This is where he encounters the strange, black, humanoid blob creature, who is sort of maliciously gleeful. At the end, the main character has to save the day by conducting music in this strange world to defeat a monster or something.
The only other detail I recall is a minor character, a young woman, maybe a violin or string player, though I don’t recall her relevance. (The main character may have been interested in her romantically? Or she may have just been a background character.)
Any and all help is appreciated! This has been driving me up a wall for years.