Most people doesn’t know about black hat SEO techniques. Black hat SEO is also know as Spamdexing.
Following are considering as Black Hat
-Keyword stuffing. (keyword stuffing include excessive use of keywords to increase SEO.
-Hidden text and invisible text
-Link farms.

Search engines takes their maximum effect to remove sites who practice these methods as these methods;
- Degrade the relevance of the search
- Degrade user experience.

So webmaster be careful when doing your SEO. Think long run. Do not use Black-hat methods. Do not handover your campaign to SEO firms who practice black-hat methods.