LF Doujinshi: A office girl gets blackmailed due to her past with her ex. blackmail, office girl, ntr, mind break

I only remember the starting pages properly, It had a office girl and then the MC comes (he was black, iirc he was from us where she was doing her studies) and remembers her. They had done a lot of things in past together and the MC blackmails her with the material he had. 

I dont exactly remember how the story progresses from that point but if i had to guess it was a something along the lines of ntr and  mind break  

Thanks!