Google has some Webmaster Guidelines. According to these guidelines search engine decides if it is black hat SEO or not. Black hat SEO users never think of user experience. They only need high rank on SERP and this high rank leads to more traffic.
Content is the king of any website, whatever it may be a blog or any business website. It plays a major role in ranking on SERP. An unique quality content with keywords always preferable for doing SEO(Search Engine Optimization).
Checking keyword difficulties for content writing is so much important. We have to choose low competitive keywords or low difficult keywords , so that there may be a possibility to rank on SERP .