Cloud gear, the missing part.

[Imladris]

To be fair, no one in FFVII really had a solid role or job. The Materia system dictated who filled what role. So if there was Darkness materia in FFVII (which there was in FFVII: BC), then Cloud would easily have been a Dark Knight.

There 'was' the Black Materia, that was used by Sephiroth to summon Meteor, in FF VII, so technically it does exist already. Plus, I thought Cloud was emo enough already (plus prior to AC, infected with Jenova cells anyway,) to be a proper Dark Knight anyway. Besides, Dark Knight doesn't necessarily mean evil knight, as look what Batman's career is like, as such.