Well, if they're blackmailing you, they went about it entirely the wrong way. If they've already sent the video, what's the threat? Its possible to recall a sent email, but if people have already opened it, they cant unsee it. Its more likely to be a phishing scam. They don't really have a video of you that they've sent (or are going to send) to people, they're just banking on you being too embarrassed by the possibility to question any of your friends about it.