I assume your question is how can I go to someone’s blog and see their originating IP.
There really in no way to do that other than hack the site and grab admin privileges. For instance, in PHP forums, the admin can see the IP address of the poster. Of course they may not be worth much if I posted through TOR or even something stronger like several proxy servers around the world. That’s assuming I could stand the painfully long lag that creates. 
 
Ok, what if I cant breach the server and achieve Admin privileges?  Well, that’s why people are getting trickier with low tech hacks.  For instance, say I take the time to host my own forum or blog. I then go  to a forum or blog that allows me to post and I can entice the blogger to come to my site and post. It’s not hard to get someone to do that actually. I actually dont even need him to post if I know my Html well enough and can just get him to click on a hyperlink! Thats usually easy enough and can give me more info than just his IP!!!  If he does post, I have his IP.  Again that IP might be worthless. I guess my point is, as security gets better and better, you will see more and more attempts to trick people to give up information than high level hacks.