Adding a prefix to the blog, but only the blog

In creating your post types register_post_type) you need to add with_front as false:

‘with_front’ (bool) Whether the permastruct should be prepended with
WP_Rewrite::$front. Default true.

so your rewrite could be

'rewrite' => array ( 'with_front' => false),

or if it’s already there with a new slug something like this:

'rewrite' => array('slug' => 'post_slug', 'with_front' => false),