Interesting question. Some blogs have the copyright symbol on the bottom of the blog. Do you need this? I looked in on a discussion about it on Blog Frog in the SITS community.
From Heather, author of Did You Remember Your Pill Today?
I just did some quick research and found this:
Your work is under copyright protection the moment it is created and fixed in a tangible form so that it is perceptible either directly or with the aid of a machine or device. The moment you write it, paint it, or put it on the internet, your work is copyright protected.
Library of Congress
In the United States, the Library of Congress officialy registers copyrights which now last for the life of the author plus 70 years. No one else can profit or copy your ideas without your permission during this time period.
Do I Need to Register?
Your works of art, music, etc, all have copyright protection with or without formal copyright registration with the Library of Congress or any other copyright office. However, copyright registration adds proof of copyright ownership and aids you in fighting copyright infringement. Copyright literally means the right to copy.
Here is some more information from the U.S. Copyright Office