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A Black Hat SEO is a practice against Search Engine Guidelines, use to get a site high ranking and Search Engines Results. The Black Hat SEO practices can seriously hurt a site's traffic credibility, and even get you banned from search engines.
Following are some of the techniques that are considered as Black Hat SEO -
Black hat SEO is a practice that increases a page's rank in search engines through means that violate the search engine's terms of services.
Black hat seo means use of unethical methods to promote your website, which simply means cheating / tricking google and other search engines to achieve higher search engine ranking and usually attain high serp ranking fastest possible, but once google identify your trick, they will ban your website from their serp ranking for ever.
Although they give chance to prove that you are not tricking serp by means of requesting them to reconsider their penalty, they rarely remove penalty.
Hence its always best to use ethical seo techniques to promote your site. Ethical search engine optimization techniques takes time to show their effect / improvement, but whatever improvement is there is usually permanent and keep on improving further.