Originally the term Black Hat meant the wicked old western movies where the character wearing a black hat did bad things, unlike the heroes who wore a white hat. The term is often used figuratively, especially when referring to the attacker who breaks into networks or computers, or creates viruses, trojans and worms.
In recent years, the terms Black Hat and White Hat have been applied to the field of search engine optimization (SEO). Black hat techniques are fraudulent (cloaking, spamdexing, doorway pages), while the White hat techniques generally follow the guidelines of search engines.
People has coined another term : Gray hat SEO. In long term, it is some what closer to black hat seo
Impact of Black Hat SEO practice
Many blogs, small to big forums, multi authored websites practice black hat seo to get immediate gratifying result (Read to make huge money). In one sentence, the end result is to get black listed or marked as content farm. If you have done SEO by a third party; got nice result ago, but now you are suffering from loss of visitors; think of getting marked as content farm by Google. Previously, we wrote how to know if your site is penalized as a content firm; you might be interested to read it as well.