Dont do it not worth the risk google will eventually penalse you. And most of it is filled with spam so why would you want it?
There are some great resources out there that are VERY blackhat (personal experience) that can stay under the radar and google will not penalise you. Most people dont know how to use these resources properly and get greedy by overdoing it. Be smart about your methods, its not always spam, those are the crappy sources.
If you are interested in blackhat seo you can find out more at this forum blackhatworld.com But use it wisely...
Comment spam is also considered black hat seo, so is keyword stuffing and link farm submissions. You can look this up on wikipedia. Also, someone else asked about link farming. From what I understand, link farming is theprocess of exchanging reciprocal links with Web sites in order to increase search engine optimization. The idea behind link farming is to increase the number of sites that link to yours because search engines such as Google rank sites according to, among other things, the quality and quantity of sites that link to yours. In theory, the more sites that link to yours, the higher your ranking in the search engine results will be because the more links indicate a higher level of popularity among users of the Internet. However, search engines such as Google consider link farming as a form of spam and have been implementing procedures to banish sites that participate in link farming, so the term link farming has garnered negative connotations across the Internet.
Black hat is that technique by doing this you can increase your SERP frequently but after some time you will be penalized by Google.