A defamatory blog posting would be considered libel (i.e., written defamation) rather than slander (spoken defamation). If a statement is, in fact, defamatory, the writer/poster potentially would be liable, but the blogger himself/herself would not (pursuant to the safe harbor provision of The Communications Decency Act, Section 230) -- unless, of course, the blogger is the one who made the defamatory statement.

Liability is a matter of state law, and standards vary slightly from state to state. Very generally, however, liability requires a false statement of fact, made of or concerning the potential plaintiff, that causes injury to the plaintiff.

While the context in which a statement is made may be relevant to determining whether it is defamatory, the standard for liability is no different (either higher or lower) simply because the statement is made in a blog posting as opposed to appearing in some other forum or media.

Although pure opinion is generally not defamatory (because it is not capable of being proved as true or false), one cannot escape liability simply by labeling an otherwise factual statement as "my opinion," particularly if the opinion appears implicitly to be based on undisclosed facts known to the speaker/writer.

In other words, the statement, "I've read his stories for 10 years, and in my opinion he's a thief and a liar," will be found defamatory (assuming it's false) even though couched in terms of opinion.

The various defenses to liability for an otherwise defamatory statement -- and the higher standard of proof required of a public figure -- are too complicated to include here.