Black hat SEO refers to a set of practices that are used to increase a site or page's rank in search engines by manipulating them. Google penalizes old-fashioned practices such as stealing content from other websites, over using keywords, and using the same color for texts and backgrounds. For that reason, webmasters should definitely stay away from Black Hat SEO techniques. The same techniques can be used by malicious rival webmasters as well. The most fitting example in this context would be spam backlink attacks by malicious competitors.