Changing URL at control panel

I see how to change the blog’s URL on the control panel. My question is this. My blog is an “add-on” domain at Hostgator. That is, it’s in a folder under public_html, named, say, blogdomain.com. The public_html root belongs to, say, mydomain.com. At the registrar, both domains use (the same) hostgator nameservers, so hostgator is handling all the juggling.

I want to make blogdomain.com “just another page” in mydomain.com instead of having its own domain name. Hence, presumably, I’d just change the URL in blogdomain.com’s control panel. I’m not *exactly* sure what the new URL should be, and I’m not sure whether I need to make any other changes. Do I need to make manual changes to mydomain.com’s .htaccess? And, beyond eliminating the “add-on” domain “blogdomain.com” from cpanel, would I need to take any other steps in cpanel?

(It is to be noted that, currently, in a browser, if one enters the URL http://mydomain.com/blogdomain.com *or* http://mydomain.com/blogdomain.com/index.php the result is NOT the most recent post in the blog!)

Any expert advice would be much appreciated. I’ve submitted this question in the hostgator forum and fetched no answer.