Void is genuinely the perfect aspect for the Black Knight. For class though I think I'd actually sway towards maid. It's my (100% objectively true) opinion that maids are the passive half of knights, both of which we see having very active control of their aspect. I think a prime example of him being a void player who actively controls void is how he refuses to accept the truth of the injury (light having themes of truth) and therefore remains uninjured (in a sense xD)