Если речь идет о site.com/subsite/
То для site.com/blog (на Wp)
Делал так
в .htaccess
# BEGIN WordPress
RewriteEngine On
RewriteBase /subsite/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /subsite/index.php [L]
