Google's informal motto - "Don't be Evil" is reflected in the principals behind Google's page ranking algorithm in that they actively try and penalise websites that engage in what some would describe as less-than-ethical practises to get their site's listed on the front page of search results.

One reason why you shouldn't employ them - it's hard to become un-blacklisted or climb back from the ranking wastelands.

What are some Black hat SEO techniques

First, the distinction between Black Hat SEO vs White Hat SEO is a bit contentious.  The reality is that in many markets, getting a high ranking will necessitate tactics that you wouldn't bother with if search ranking wasn't important.  Such as article and directory submissions.

In my view, Black Hat SEO are techniques used to improve search engine results page (SERP) performance in a way that leads to a bad outcome for users and the internet in general. Ones that break search engine rules and create poor user experiences. Automatic website creation, article spinning and blog comment automation are some examples because the end result is rubbish sites with duplicate, poor quality content.

While not 'illegal' or even against anyones guidelines, these techniques are frowned on by search engines and many in the SEO community.

The problem is they may well work in the short term.  But if you are contemplating paying someone to improve your SERP ranking and they employ any of these techniques you risk having your site disappear from the search results when Google catches up and changes it's algorithm.

Even if this doesn't happen, the results are not sustainable so you're likely wasting your money anyway.

These techniques include:

• Keyword stuffing - where keywords are repeated throughout a page, ususally out of context or with no other content. Sometimes the keywords are completely unrelated to the 'real' content, but are ones that people will click on (I'll leave what these words are up to your imagination).
• Invisible text - adding keywords in ways the user can't see them.
• behind images or as white text on white backgrounds
• tiny text or text embedded in the html code.
• Cloaking or shadow domains and doorway pages - sites or landing pages full of keywords but which redirect the user to another page or site.
• Maybe not black hat, but of highly dubious value is submission to "100's" or even "1000's" of directories.
• Setting up fake social network profiles
• Social media spamming
• Cybersquatting or domain squatting
• Blog comment and trackback spam (eg pointless 'great post' comments)
• Multiple domain name registrations pointing back to a site
• Link farms - networks of 'fake' sites, particularly blogs with blog link lists. This is incredibly common and we see several examples here in NZ.

Post Penguin and Panda updates to Google's algorithm, site owners are now wondering how to get rid of back links to their site that are coming from 'bad neighbourhoods' or otherwise hurting their ranking.  This is hard to do.

This article that explains in detail what can happen when Google decides to penalise tactics like link farming.

Good SEO takes time and effort to do and to get results.