Thanks lucy. That makes a lot of sense to me.
My worry was that when Google came looking for the sitemap, at website.com, it wouldn't find it, so I was trying to tell it [Google] to look at blog.website.com for it. :)
The blog is powered by Wordpress, which uses a plugin to generate, optionally compress and list the sitemap, so it sits at the blog.website.com UR, not in the root.
The root and the blog subdomain run seperate robots files.
I will remove the .htaccess entries and see what happens.