Geat article here is the Wiki definition.

Black hat
From Wikipedia, the free encyclopedia

Originally all people with a high level of skills at computing were known as hackers. Over time, the distinction between those perceived to use such skills with social responsibility and those who used them maliciously or criminally, became perceived as an important divide. Two terminologies developed: the former became known as hackers or (within the computer security industry) as white hats, and the latter as crackers or black hats. The general public tends to use the term "hackers" for both types, a source of some conflict when the word is perceived to be used incorrectly in written reports. In computer jargon the meaning of "hacker" can be much broader.

Usually a Black Hat is a person who uses their knowledge of vulnerabilities and exploits for private gain, rather than revealing them either to the general public or the manufacturer for correction. Many Black Hats promote individual freedom and accessibility over privacy and security[citation needed]. Black Hats may seek to expand holes in systems; any attempts made to patch software are generally done to prevent others from also compromising a system they have already obtained secure control over. A Black Hat hacker may have access to 0-day exploits (private software that exploits security vulnerabilities; 0-day exploits have not been distributed to the public). In the most extreme cases, Black Hats may work to cause damage maliciously, and/or make threats to do so as blackmail.

Here is a few more resources : edu links : hidden cloaking : blogger links

Forums and blogs are rife with this at the moment and the only answer seems for forum owners to move the spam to a controlled area.