Total Recommendations: 3
Though they're both thrust into dangerous, unfamiliar situations, the protagonists in both anime are similarly detached from their predicaments. They're also of a similar age group (14/15). Recurring characters include... the older, concerned guy who starts to act like their mother; the spunky and dominating "girl warrior" type.
Similar setting: rich, somewhat arrogant noble kids, older (and hotter) servants, Gothic styles. Both try to present a fantasy world that resembles a historical European setting to various degrees, but with the critical aspect of the story based on a supernatural twist. In Kuroshitsuji, it is the concept of demons and angels. In Pandora Hearts, it is the "Abyss," a dimension where time is warped. Both protagonists have been bestowed with a sort of life-binding curse that they view as a punishment, yet ironically neither seem to value their lives very much.
The most obvious resemblance is their music composer, Yuki Kajiura. The musical style she uses in both anime are very similar, dominated by emotional piano pieces, violin solos, techno beats and haunting vocals.