Okay, so I think I got it, reading the support page very carefully, but I'd love to make sure.
http://en.support.wordpress.com/domains/map-subdomain/ is the page that I read very carefully. The very last part "Pointing a subdomain of a domain mapped to WordPress.com to an outside website" is what I need help with.
I'm trying to use WP as my "site" and Blogger as my "blog". Meaning I want blog.whatever.com to belong to Blogger. And of course whatever.com (example) belongs to WP.
Does this mean that after purchasing a domain and mapping from WP ($18), I'd have to purchase an additional mapping upgrade for blog.whatever.com ($13)???
The blog I need help with is damiandownfall.wordpress.com.