Ya that sounds true for the most part. What made it unique though in the movie musical genre? the story?
The biggest difference in Sweeney Todd than most movie musicals is how its presented. All movie musicals are very bright and use stories that are happy, colourful with some romance.
Sweeney Todd was the opposite to all of this, it was dark, scary and its about getting revenge while killing many others. In Sweeney Todd, they actually used the above tones of the normal movie musicals and used it for comedy value.