titles says everything
though he doesnt have to stay by her side all the time. just listen to her demands or so.
id like the blackmailing girl to have a reason like, needing him to change or protect her or etc.
and id like the boy to be angry at her for blackmailing him (at the beginning at least)

preferable shounen seinen but anything goes

Watashi ni xx Shinasai! and try the blackmail category. That should do the work for you 😀

Help! I'm looking for... manga with dangerous guys [url]http://www.mangaupdates.com/showtopic.php?tid=39874&page=1#post594033[/url]

The only one i can think of is Aku no Hana but I think the boy's sort of a wimp. Dunno if you'll like it but worth a try.

[CENTER][img]http://www.wizards.com/magic/images/whatcolor_isblue.jpg[/img]Take the Magic: The Gathering 'What Color Are You?' Quiz.[/CENTER]

“I don't know that love changes. People change. Circumstances change.” - Nicholas Sparks

It's more a business contract than blacmail, but Limited Lovers fits the requests.

Ore no Kanojo to Osananajimi ga Shuraba Sugiru
Lossely translated, the title means "Being stuck between my girlfriend and my childhood friend is Asura Hell". The story is about this boy, Eita, who has refused to get into a love relationship with any girl in school because he wants to concentrate on his studies. One day, his classmate, a beautiful girl with silver hair, Masuzu, suddenly asked him to be her boyfriend, shocking the entire class. When they are alone, the girl revealed that she wanted him to be her pretend boyfriend so that the other boys will stop pestering her with love confessions. Masuzu believe that Eita is just like her in terms of how they view love relationship, and therefore she knows he will not betray her. When Eita refused, Masuzu revealed that she has stolen his diary, and will leak the notebook to the public if he refused. (Blackmail)

Btw, don't just search for tags with blackmail. You should try "fake relationship" also for better results.

Nozoki Ana a.k.a A Peeping Hole

Kido, a freshman university student, has just move into his new apartment in Tokyo. On the first night, he discovered that there was a hole in the wall. Looking through it, he saw a girl/his neighbour half-naked and pleasuring herself. Suddenly, she disappeared from his view, and when she reappeared, he realised that she was looking through the hole at him. He immediately went outside and knocked on her door, hoping to apologise and inform her about the hole in the wall. However, one thing led to another and he accidentally trip and landed on her.

The problem was, as he was trying to get up, the girl quickly snapped a photo of him in that incriminating pose with a digital camera. Thereafter, she introduced herself as Emiru, and revealed that she gets turn on when she's being watched. Next, she said she will agree to delete the photo, but only if Kido will agree to her condition; they are to keep the hole undisturbed, and use it to peep at each other, showing their "honest" self to one another.

Now, if you think this is a H-manga and quickly dismiss this title, then you will be making a big mistake. This manga has a lot of depth and character development. I only recommended this cuz you said Seinen is fine. If you're willing to give it a chance, you'll get your reward in chapter 44.

I second Ore no Kanojo to Osananajimi ga Shuraba Sugiru.

Ibitsu (OKADA Kazuto) Dude gropes a girl, Girl stalks him, blackmails him and does some S&M things to him. Awesome stuff.

[img]http://img803.imageshack.us/img803/6167/avvy.png[/img]
Rape is imminent.

Peach Girl?

Velvet Kiss fits. It is borderline H though.. but you did say anything and this is seinen

maybe.. S Senjou no Tena? However, it's much more like he is forced to stay with her rather than she blackmailed him. I'll put this here anyway because he was forced to become her "pet"(typical, I know.) because she needs something inside the guy and if she took it immediately he'd die. It's said at the first chapter so I don't think that would count as a spoiler

“I thought also of another thing between us. Call it trust... That is one of its names. It is a very great thing. Though each of us alone is weak, having that we are strong, stronger than the powers of the dark.”