No apologies needed.  This fieature isn't a part of the blog module, so you would have to modify the code to track this information.  Alternatively, you would have to add an aspx page to your site which would function as a server side IP address tracker and then write some JavaScript and attach it to the click event of the button used to send a comment which would do something like set the src of a small image be be the URL of the custom IP Address tracker.  In this page, you would use code to track the client IP address along with the referring URL to keep track of the blog entry to which the comment was added. 

Of course, as you mentioned, you would need to realize that you are most likely crossing a line there which would be easy for site visitors to see by viewing the HTML for your page. 

HTH,

Don