What is a black hat technique? Well there are two types of techniques when trying to improve your search engine rankings. If we use a star wars analogy white hat techniques are the light side, ways of improving your search engine rankings while adhering to search engine rules/policies and focusing on a good user experience. Black hat techniques are the dark side, ways of improving search engine rankings in an unethical manner, often breaking search engine rules and not focusing on the user's experience because you are trying to manipulate website rankings.
Often using black hat techniques will give you short term gains in search engine rankings, yet ultimately that Google hammer will come crashing down on your head! Black hat techniques used to be a legitimate way of improving search engine rankings, but over time these techniques have been abused and over used and they are highly frowned upon in the SEO community. The main reason I would advise to stay away from black hat techniques is not because Google will find out and crush you (because they will), it's not because it ruins the users experience and it's not because it's immoral. It's because SEO is a long term endeavour, not something that is going to be solved overnight with a few dirty handed techniques (of course the other reasons are reason enough not to do it).
Here are the techniques to stay away from:
Keywords are important for rankings, it helps your website get found by matching peoples searches to the keywords in your website. There is no magic keyword density percentage to have in your website, you simply have to look at it from a user's perspective. Read your web page, if when your reading it, keywords appear too frequently, seeming unnatural, then you probably have too many in there and search engines are going to take a dim view. Sometimes people will try and get around this through hiding the keywords, so they aren't seen by the user, however search engines will find them and punish you severely for using them. Remember it's about the user's experience, focusing on good content is the key, worrying about keyword density is a thing of the past!
Again links are important for improving search engine rankings, but again people have found ways to abuse the system. Google are very smart and very unforgiving and you will be punished. Link farms are essentially "you scratch my back I'll scratch yours", they are a group of websites that all link to one another in the group. Obviously you're going to get a few reciprocal links, but abuse link farms and Google will most likely punish you.
Again link buying is abusing search engines fondness of inbound links, buying links is simply paying for another website to feature a link to your website on their website. This is a very unethical way of building links and as well as that very expensive. On poorer websites you can expect to pay anything from £100-£500 depending on the website, but on websites with higher domain authorities you can expect to pay monthly subscriptions of about £100 a month! Extortionate when you think Google may catch you in the end anyway.
Doorway pages are web pages that are designed to fool search engines, they are optimised to rank highly for a typical keyword or phrase, then when the user clicks on the link it quickly redirects the user to a different website (the desired website destination). Doorway pages are often of very poor quality built to manipulate search engines and not built for human use. Again Google don't take kindly to this dirty handed technique, expect them and sites linked to them to be taken out of Google!
Blog comment spamming
We have all seen it, you go on a blog and instead of seeing healthy discussion you see shameful comments with links back to their websites. It looks like a desperate attempt to get a link back to your website (which it is), and if done to excess may raise red flags for Google.
Good content is important for your website rankings, especially since the Panda update from Google, which has seen a shift away from keywords and links towards quality original content. Notice original is in bold, if you copy content from other websites to put on your website Google can tell and this will affect your rankings! It's best to do your research and come up with original industry specific content regularly.
So be careful when it comes to you SEO practices, remember to focus on the overall user experience and regularly uploading quality, original content. It's a hare and tortoise kind of race, there is not a quick fix! Don't join Vader on the dark side!