The term “black hat SEO” refers to dishonest tactics designed to get sites good rankings in the search engines, but deteriorate the quality of the search engine results pages. Search engines are primarily an advertising medium for companies. The job of a search engine optimizer is to get companies ranked high in the search engines. This means using tactics that work – black hat, white hat, no hat, whatever. The responsibility of a search engine is to maintain the quality of its results. If you are using tactics that are considered “black hat,” it only means that the search engines don’t like them, and you could be penalized for using them.
Classic black hat tactics usually aren’t effective anymore. Among them are mirror sites, gateway pages, and keyword stuffing.

Mirror Sites

Mirror sites have the same content under different domain names. Google is pretty good at finding duplicate content. If you use mirror sites, Google will pick the domain it deems to be more important and only show results from that domain. This can cause problems because it might not be the domain you want to show up. Yahoo! isn’t as good at finding duplicate content, especially if you spend a little time being tricky. However, competitors can still complain to Yahoo!, and if your violation is particularly egregious, you could be removed from their index. Most sites won’t link to a mirror site – it will definitely limit your presence on the Web. Mirror sites are the oldest trick in the book, and they aren’t worth the time.

Gateway Pages

Gateway pages are optimized pages from different domains which lead to the main domain. They are supposed to get sites multiple listings for the same keyphrase by using good on the page optimization. This tactic doesn’t work well because link popularity factors are important to rankings, and it’s difficult to get links to a gateway page. It’s possible to “cloak” gateway pages. Cloaking means showing a search engine a different page than a visitor. There are different ways to do this, but it’s a lot of work and a page that is cloaking runs the risk of being found. It’s not worth it.

Some people might ask, why not optimize the pages under a single domain for the same phrases and have multiple pages show up in the results naturally without using gateway pages? This is much harder than using gateway pages, and it isn’t particularly attractive to visitors. Most keywords a site wants to show for are very closely related. Creating separate pages targeting each of these terms can be repetitive for site visitors.

Keyword Stuffing

Back in the 1990′s or so, putting large amounts of keywords into tags would get a site ranked well. This no longer works. Putting keyphrases into pages is important (even though it is sometimes very unnatural), but the keyphrases have to be visible to the user. Over-use of keyphrases won’t help you. Google appears to use keyphrases in body text mostly to determine what topics the page is relevant to, and not as much for judging the importance of a site. Yahoo! gives more weight to this kind of content, so technically there may be some optimal use and frequency of keyphrases for a page to get it ranked high. It is doubtful that this number is universal for all terms, and this also doesn’t take keyphrase proximity (where these phrases are located relative to each other) into consideration. Keyphrase density (the number of keyphrases per total number of words) is a useless number. Keyword stuffing is not an effective tactic.