What about creating an ACTUAL Dark Knight?

[Reynhart]

Nope. I've played FF IV a LOT and Cecil doesn't do this. He casts some damaging spells (Fire - For example) but that's it.
He uses "Black Magic" hence Dark. When he becomes a Paladin he uses "White Magic" that's all it is.

No, Cecil doesn't use any Black Magic. However, he uses Darkness/Souleater that sacrifice its HP...unless you've played FF IV Easy Type, that removed all of his skills.