RewriteEngine on
RewriteBase /

#if not already blog.website.com
RewriteCond %{HTTP_HOST} !^blog.website.com$ [NC]
#if request is for blog/, go to blog.website.com
RewriteRule ^blog/$ http://blog.website.com [L,NC,R=301]

RedirectMatch ^/subdomain/(.*)$ http://subdomain.mysite.com/$1