Somewhat of an open question really. I think for all of blogging we have concluded that the words in your blog post were in fact written by you. I see two reasons plagiarism is bad: both and injustice to whom wrote the original but also a false indicator to your cognitive and creative ability.
Well, I think it is more challenging than that. Even if the author writes the piece himself, most professional writing is subject to editing, which can substantially change the finished product. The one thing we can be reasonably, if not entirely, certain of is that the piece is one the author is comfortable endorsing as his own.
