metaljoy on "subdomain / domain mapping"

I want for the url blog.metaljoy.com to point to metaljoy.wordpress.com. I added a Cname record to the metaljoy.com domain.
I paid for the upgrade and added the metaljoy.com domain. I hope this is correct. My concern is that metaljoy.com will be redirected to metaljoy.wordpress.com when what I want is the subdomain blog.metaljoy.com to be redirected to metaljoy.wordpress.com.
This is what I intended to do when I added the metaljoy.com domain.