Kuroshitsuji, Karneval (TV) Recommendations
In both series, there's a fantasy like feeling with a more sophisticated style of its artwork.
The main male protagonist is a young boy; although they have very different personalities. However, it focuses on his relationship with others as well as the people he meets along the well.
Both series also gives off a gothic like feeling with its artwork and also a small sense of mystery.
Black Butler is considerably more episodic and most likely the more dark of the two, but Karneval is not far behind and is surprising in a few different episodes. Both are punctuated by similar sorts of comedy and include the occasional hilariously dorky character.
Both have similar character designs and gothic art styles.
Both feature primarily male casts with debatably BL (though never over-the-top) undertones.
Overall, if you liked Black Butler, you are likely to enjoy the darker plot aspects and animation style of Karneval.
Both have the main character trying to find out some information about people they care about. Both have similar art, and bishies. They have the same atmosphere, which is serious but with random comedy thrown in.