It starts with a girl working late, goes into her boss' office only to find him masturbating! She thinks she got something to kind of 'blackmail' him over but he just says no one will believe her. Further in the stort, they end up together. The art of the manga is gorgeous! And he has black hair
It's a yaoi manga about a singer and a business man.
The business man moves into a new place with his cat - when he goes out on his balcony he hears his upstairs neighbour singing (really good). Later his cat sneakes into the upstairs neighbours apartment. Later in the story, we find out the the upstairs neighbour is being exploited by another singer who takes credit for his work
I think it's "The siren's song".
Try it.
That's the one!! Thank you so much!!