Black hat link building tactics are techniques used to drive traffic to a website by exploiting website loopholes, enabling a site to rank higher than it should through "organic" search means. Search engines like Google have become aware of such practices to game the system and boost search rankings. Sites like Google have developed algorithms to penalize this kind of activity, so some of these link activities aren't as powerful as they once were.
Black hat link building is not designed to serve the user experience but instead simply to boost page rankings for a site by gaming search engine optimization rules. One strategy of black hat link builders is the spam-oriented tactic of posting comments and a link to the site on other third-party sites. Generally, the link has little to do with the content on the page in which it is listed.
Another tactic black-hat link builders is creating link farms. A link farm is a website that features a series links to other sites without any content with inherent value on that page. Some link farms sites look legitimate but ultimately are efforts to drive traffic to certain pages without producing any value on their own.
While these strategies can drive some traffic to a site, the links aren't organic and search engines like Google will won't see append value to these links and, hence, the page. Some of these tactics may produce results in the short-term and some may even go unnoticed for a prolonged period of time, but in the end Google will continue to update its algorithm and eventually expose and punish such shady practices.