who the hell is the black haired dude

This is just my guess, so it's technically not spoiler. It's most likely Azazel the male lead, or his past identity as a fallen angel. His right blue eye turned a bit green when he first saw the FL. And the FL is probably his past lover too since she kept dreaming about him or recalling memories of his past self.

you’re right, its azazel from 1000 years ago. (source: novel updates spoiler)

You should’ve said spoiler at the beginning of your message :/