Black Hat SEOs use illegal methods to get your page to rank high. But they reap only short term benefits for your company. In the long run, it may lead to penalizing or even banning your page. If they guarantee results in a short span of time, and are less expensive than others they may be a shady SEO in web design Coventry. Any company using techniques such as bookmarking, article spinning, guest posting may give you negative SEO.