Unfortunately, some forum participants make comments in forums only in an effort to publish links back to their own sites in signatures. This may be acceptable if the user provides help or assistance to another member but many members just gives no relevant content and increase no of posts and backlinks in there signatures.
Probably not so effective, as well maintained forums remove useless links.
Duplicate Domains/Content
More than one domain contains the same content. Now the question arises what is duplicate content. If I display a news ticker from BBC is that considered as duplicate because there may be lots of webmaster who has the same ticker. No its not duplicate but if I copy the same file to two different domains thatís what is duplicate content.
I am not so sure if you should call this black hat, as there are ton of reasons why duplicates exist. Also copying and pasting is one major contributor to the creation of the web, IMO, so it's tough to debate on this one.