This worked for me. Might be of some help for others.
<VirtualHost *:80>
RewriteEngine on
RewriteCond %{HTTP_HOST} ^blog\.example\.com
RewriteRule ^(.*)$ http://www\.example\.com/blog/$1 [L]
ProxyPreserveHost On
ProxyPass /blog !
ProxyPass / http://localhost:9000/
ProxyPassReverse / http://localhost:9000/
ServerName example.com
</VirtualHost>