The Kuroshitsuji/Black Butler manga is a bit darker and less angel-infested than the first anime season. (There is also a marked lack of giant hell hounds. I feel that this is generally a good thing as I can’t stand Pluto.) The relationship between Sebastian and Ciel is a little more adversarial–in the first chapter Ciel sends a martial artist after Sebastian, and then gives him lemonade that Tanaka made with MSG instead of sugar. Also in the manga for the first few issues, he has a nearly perpetual evil smirk. (Ciel in the anime is a much more somber kid, though he has his evil smile moments as well.)
In the first chapter, Ciel deals with a dishonest employee and Sebastian manages to overcome the obstacles of Maylene, Finni, Bardroy and Tanaka. In the second chapter, Ciel’s fiancée comes over for a visit and manages to break his signet ring. In the third and fourth chapter, Ciel gets kidnapped by an Italian Mafioso and Sebastian comes to the rescue. The fourth chapter includes a scene that does not cross over into the anime–where the Mafioso threatens to sell Ciel to a pedophile. (The manga is definitely not for younger readers.)
Aside from the darker tone, the manga is also much freer with the anachronisms than the anime. (In chapter three one of the thugs is using a cell phone to talk to his boss and in the first chapter the conniving employee gives Ciel a videogame.) I initially had a big problem with the anachronisms (as I have mentioned elsewhere) but after a while it doesn’t seem to matter as much. (You just have to up your dosage of Suspension of Disbelief.) There is very little difference between these first chapters and the corresponding episodes of the anime, though there is definitely a greater sense that Ciel is in a great deal of trouble in the third and fourth chapters. While the anime was what drew my interest, I feel that the manga is definitely better than the anime as far as characterization and plot go, while the anime is generally pretty good throughout most of the series just before it diverges from the manga.
What mainly draws the attention is the relationship dynamic between Sebastian and Ciel. Sebastian in the manga seems to have to work at not laughing at Ciel’s emotional dependence upon him. (There are several scenes where Sebastian is posed with his face half concealed, and you just know he is hiding a smirk.) Ciel in the manga appears to be aware that Sebastian is laughing at him, and just as obviously does not care. This aspect of the relationship is more clear here than in the anime. (The reactions and interactions are much more muted between them in the anime, or so it seems to me.)
Ciel deliberately tests Sebastian in both the anime and in the manga. For example, in the manga, he hires a martial artist to attack Sebastian, and later lets Sebastian drink lemonade Tanaka has accidently tainted with MSG, in the anime Ciel has a tendency to throw things at Sebastian. It is less clear here on whether Sebastian tests Ciel the way he is tested, though it is clear that Sebastian expects and requires a certain standard of behavior from Ciel. (These expectations are usually set within a framework of “upper level servant/retainer who is instructing a much younger superior in how to act like an adult.”) Within the series, Ciel might make a few half-hearted attempts at subverting the instruction but in the end, he is shaped by it.
The general tone of the manga throughout the series ranges from dark and serious to dark, and funny. The humor is generally ridiculously slapstick, which should be jarring next to the horror/mystery storylines, but here it mostly works. It helps that it is eventually revealed that the “Silly Servant Trio Plus One” are all some variety of Crouching Moron, Hidden Bad Ass. (What I found the most interesting about the *Silly Servant Trio Plus One is that the transition from Bad Ass and back to Moron is pretty much seamless and there is never a sense that this is anything other than their proper character whether they are in Bad Ass or Moron mode.) Another factor in why the occasional slapstick is effective is that there are so many anachronisms and things that don’t make sense that you end up accepting the slapstick quite readily after a while. (Though there are a few moments in the manga, I find extremely annoying or unacceptable.)
*I do not include Tanaka in the Trio because while he is very silly, he is apparently secretly formidable enough that Sebastian actively respects him and moves around him. Also, he was a member of the original serving staff, where the Trio was hired after the fact.