Someone is harassing me online thru a blog. How can I determine their location from the blog's IP address?

Try using a packet sniffer.  But doubt you'll get your information

YOU can't but the FBI CAN so call them

you can't. that's what the police are for.

The IP of the web site will only take you to the location of the web site.  It will not take you to the location of the computer of the person who is harassing you.  There is no way to get that address from the site's IP.

If they are harassing you on the web site, contain the moderator of the web site.  They will be able to block that person.

In spite of what they show you on the TV programs, you can not find a person's physical location through their IP address.  The IP address will only take you as far as whoever their service provider is.  Beyond that you are blocked from finding the actual location of the person, their name, etc. 

The ISP is given a block of IP addresses.  Those are handed out as people need them and freed and reused when they disconnect.  So you can not get to a person through through their IP.  The ISP would have to look back in their records and find out who was using that IP at that time.  That takes a court order.

Now, the service provider will give you some idea where the person is located.  If the service provider is in San Francisco, for example, odds are the person is also in San Francisco and not in New York or Tampa.  But that is about as close as you can get to them with an IP address.