Trinity Blood, Owari no Seraph Recommendations
These manga feature similar theme: vampires and humans fighting against each other in a dystopian world. Also there are characters that are very attached to their friends, best friends turning into enemies and vampires working along with humans for the good of both species (enemies becoming friends). TB is more adult-like, more mature and gory while OnS focus more on younger characters, "school" settings, but it also provides awesome action.
I personally like TB more because it has a lot of political plot and great fights, also slight romance while OnS focus more on friendship and only in developping the three main characters. In both