Possibly adding /blog based on http_host within rewrite conditional?
In the http protocol the http host is the domain part of the URL. You cannot add a path to the http host.
Domain mapping doesn't support what you are proposing because DNS lookup & the way the web works doesn't support it.
If you have WP installed in the root of your site then you can set up your mapped sites so a request for the root of the domain redirects to a /blog page within the site. You can also should be able to add /blog/ to the beginning of your permalink structure.