Theirs two basic Black Knight types in Final Fantasy - Rogue Knights (What we got) and Knights who use the power of darkness (Which doesn't necessarily make them 'evil').
I was hoping we'd get the FF4 type of Dark Knight - a class where one risk their soul for the power of darkness, which they either bow too, or try to tame and use for good purposes. All the Cecil gear got my hopes up, but instead we got, as others said, Batman. This version of the Dark Knight is, of course, completely valid and in line with Final Fantasy. It's just not what I was hoping for.