I hesitate to venture too far here as I am not a Wordpress Pro but I have not seen a plugin that would require admin approval prior to allowing registration. Of course you can keep guests and registered users from leaving public comments until you approve the post.
There are plugins to prevent registration by
IP address and plugins to allow
finer control over registered users access to posts.
CMS systems, such as Joomla, can be enhanced to prevent registration unless the admin approves the request. I use this on a family website to prevent non-family members from seeing family content.
Kind of depends on how you use the blog as to what your options might be.