[Iro Murasaki (Kuzui)] Majime Sugiru Senpai wa Service Zangyou de Asa made Ikasareru | The Serious Senpai Works Overtime As She Is Made To Cum Until Morning [English] [Penguin Piper]
Ugh. Yes, this is NTR but the story is so poorly written that you could also say that it isn't NTR. The purpose of NTR is to increase the girl's sexuality, but does that really happen in this story? The author created a kouhai character who is so freaking stupid that he doesn't question why his senpai is going to an important business meeting dressed in CASUAL CLOTHES. Not only that, but she's dressed like a prostitute. The sex scenes with the client isn't enhanced in any way because the girl's intentions were shown right from the beginning. Even if you remove the kouhai from the story, it doesn't change the way you look at the girl from the start (i.e. She's a slut who trades sex for contracts.)
A better way to write the story would be to build the girl's character to show that she is a highly respected professional (which includes dressing properly). This would give the kouhai's respect and admiration for the girl actual meaning; The girl's sexuality is enhanced in the sex scenes because it looks like she is doing something taboo.
This isn't NTR since she isn't cheating on anyone or being stolen from anyone. I also don't think this is blackmail since it's more like indirect prostitution. Still pretty sexy