sorry, can anyone enlighten me, is the emperor and black haired guy is one person?

Idk I don't think so tho

For a moment I thought you were talking about the red haired guy who is connected to the black haired guy's body. But to answer your question...No.

The emperor is a SEPERATE character plus based on what I saw of the raws he gets his own love story (A guy of course! This is bl even with the annoying Chinese censorship) alongside the main couple of this webtoon.