!∀black thing ∃ raven-nature

That's where I don't follow. I read the original sentence as "for every x there is an y such that the relationship P obtains between x and y". I'm OK with your assigning "black things" to x but "raven-nature" needs explanation; I don't see how to parse it as a relationship between two things previously introduced.

The edited version makes more sense to me now.