Blatant manga adaptation. Unless you're completely oblivious of the manga/anime scene, it won't take you long to realize that there's an illustrated franchise backing this film. The styling, plot and reveals are all pretty outrageous, but that comes with the territory. The upside to all this is that Black Butler had a pretty sizeable budget to work with.
I wasn't familiar with the franchise, but no worries, the film gives you a short introduction that should get you up to speed. There's still a lot of nonsense you have to take for granted (demons, French maids, revenge ploys, ...), though that's also part of the appeal. It's nothing too original, but the lore is solid enough.
The action is surprisingly slick, the cinematography looks lush and the styling is on point. The film has some pacing issues, there's a bit too much dialog and there are moments when it takes itself just a little too seriously, but this was a pretty fun and well-made film. I'm a bit surprised this isn't better known, as this clearly wasn't a minor production.