Setting only a single site (on a multisite setup) to the subdirectory /blog/

I’m going to be as clear as I can here. Sorry in advance if it seems a bit of a jumble, I’m quite a bit confused myself with this situation.

As it stands, my client has several established sites setup on a WP multisite at site.com in a subdomain setup. Some of the child sites have their own domains setup (anothersite.com, site2.com) and one or two primarily use the subdomain (abc.site.com). Of course site.com is the main site.
That’s all fine and needs to stay that way.

My client has set up a child site as the official blog for the company at content.site.com, and is asking me to make site.com/blog redirect to this child site. Additionally, site.com/blog/permalink/to/post should serve up content.site.com/permalink/to/post.

I’m fairly new at this kind of dilemma, and at first I thought that I would need to set up subdirectories somehow, but that wouldn’t be desireable for several reasons. Now that I’m writing this down, it seems that I’ll probably need to add a rewrite rule to .htaccess, but I’m hoping to get a bit of insight on how to do this and what rule this might be. Note that I don’t have access to root, just to /wp-content/

Something along the lines of
RewriteRule ^blog/(.*)$ http://content.site.com/$1 [L,R=301]
would probably do the trick… is that all I need to do?