Responses
Tbh, yeah, I don't think Seonwoo said he was going to blackmail him, but he did try to rile him up about the notification: https://www.mangago.zone/read-manga/punch_drunk_love/uu/to_chapter-4/pg-14/
In any case, Taemoon's right in calling him a stalker because that's basically what Seonwoo is, lol. But blackmailer? Not really.
Did Sunwoo never try and blackmail Taemoon? Like I don't remember any blackmail there at all but Taemoon constantly brings it up. He just jumped to his own conclusion over Sunwoo bringing up the app stuff. There was a whole chapter basically dedicated to clearing the miscommunication between them but that was never addressed? Am I going insane or something lmao