htaccess redirect domain a.com to b.com/blog with keep a.com adress

I want redirect domain a.com to b.com/blog
but want keep a.com adress in browser bar.

I currently have

RewriteEngine On
RewriteCond %{HTTP_HOST} ^a\.com$ [NC]
RewriteRule ^(.*)$ b.com/$1 [L,R=301]