Mod_rewrite将子目录定向到另一个URL

I am trying to play with Apache Mod_rewrite and I cant get this to work.

I want to redirect

xyz.com/blog to xyz.wordpress.com

I edited the .htaccess file to add this line.

RewriteRule ^blog/?$ http://xyz.wordpress.com/$1 [NC]

But it doesnt work.

Try this one.

RewriteRule ^blog/(.*)$ http://xyz.wordpress.com/$1 [NC]

Also don't forget to add

RewriteEngine on

on top of RewriteRule.