Erik J. Heels explains clearly and succinctly what Fair Use is.
For blogging, Fair Use would be using maybe a few paragraphs, then adding your own commentary. You can also just re-phrase what the author said (with a link back), then Fair Use no longer applies.
With a little thought and effort, Fair Use need not be an issue for blog posts. Plus, the less you quote, the more the blog post becomes yours. It then becomes your voice, not the quoted voice, which is what blogging is about anyway.
Tip: The Big Picture