If you search DP, you'll find a million other explanations of what blackhat it. Or even Google it. One of the first results gives a fairly good explanation: http://websearch.about.com/od/seononos/a/spamseo.htm
Are you talking about this? Okay.. Its a kind of techniques used in SEO and is not supported by Search Engines.
Blackhat is an SEO trick/technique which involves tricking search engine's to get higher serps. e.g cloacking a pr9 domain, so your domain looks like a pr9 on the toolbar, or... using doorway pages ect.. stick to grey/white hat