Once someone posts a blog, is it automatically copyrighted or can any reader use their content?

Yes, it is automatically copyrighted as soon as it is written. To present their information as your own or copy large portions of it without permission is illegal and unethical. You can, however, quote some material for some purposes such as "criticism, comment, news reporting, teaching, scholarship, and research" (title 17, section 107 of U.S. code). There is a lot of room for interpretation, though, so check out the following two links:

Copyright FAQ:
http://www.copyright.gov/help/faq/

Fair Use:
http://www.copyright.gov/fls/fl102.html

And of course, make sure if you quote a source that you cite your sources (in my case, the above two links).

According to U.S. copyright law, any copyrightable material is immediately considered to be under copyright protection the moment the copyright symbol © is placed on it, whether that material has been registered with the copyright office or not. So, if you see the © symbol on the blog, the material is protected by copyright. The question of whether you can reuse the content depends on what rights the copyright holder reserves when they copyright their material. Read the copyright statement on the blog (if there is one) and/or e-mail the webmaster for permission to reuse the content.

EDIT: KarmaKitty is absolutely correct. Copyrightable material is considered to be under copyright protection the moment it is created, not when the © symbol is placed on it. However, the entire point of registering copyrighted material with the government is to provide yourself with a more stable basis for demonstrating your rights to the material in court. A copyright means only that you own the rights to the material, not that you have denied others the permission to copy or distribute that material. If the author does not reserve those rights for themselves (by placing the © symbol on their work, for instance) the material isn't necessarily off-limits. KarmaKitty is correct, but for the purposes of this particular question I stand by my previous answer.