Great question,

Black Hat SEO can be summarized in two distinct fields. The first field is a new idea and the search engines have not disproved of it yet. The second is the complete disregard to those guidelines. The issue with these methods are that, you the client, take chances on doing something that will get you banned form the search indexes. This means that your site is not found on the banned engine. For instance if you get banned on Google. It is as if your site does not exist. Black Hat will eventually get you banned some techniques do get adopted into the White Hat SEO, but they are proven safe such as the dash instead of the underscore.

Black Hat is more of the dark side of code manipulation. I know it, I understand it, I recognize it and inform my clients of possible issues that will arise from such tactics. I will not turn you in, but I will not associate with any projects that use Black Hat SEO. Ethically speaking I can not associate with these techniques, they will in the end cause more damage to a site then help it.