Simplified, Blackhat is valid usable content generation, usually automated.
There are generally two methods you can go about doing blackhat seo, first is spamming other sites with links or generated content… And the second is to generate content, and tons of it. Autoblog’s, comment spam, email marketing, message boards, and social networks (myspace, facebook, youtube etc.)
However if you spam sites with your chosen keywords in order to rank higher in google, you will see a short term jump in rankings, but this will be gone in less than a month due to google’s algorithm picking up on those spammish links you’ve been dropping or automating… which leaves you back at square one. Your not gonna keep buying the bitch drinks if she aint going to put out. So the best method for doing this, which has been tried and tested, is to generate comedy/humor websites based off of feeds and databases that have several entries so that the output is relevant to the keywords for titles and meta tags. I setup a site similar to this and spent less than $50 and in about 2 month’s time I had earned almost $2000, blackhat at its finest.
Personally I don’t think people should sell SEO, but practice it on their own and learn. SEO is an ever developing realm, with the algorithms constantly being updated and changed.. The top blackhats stay ahead of the algorithm either by getting the code or by guessing which methods would go undetected. There are many outdated methods (keyword stuffing, invisible text, doorway pages etc.) that used to make a lot of money, and in order to stay ahead you have to constantly be on top of what works.
While blackhat SEO is unethical, if used properly in moderation (key word here) you can make a pretty penny, and at the same time not get penalized by the search engines as long as you stay within some very specific boundaries, and dont get greedy.