Permit access to your blog only to the logged in users

if a user is not logged in, redirect to site specified site or show page with user defined message.

This plugin displays a blank page for all unauthenticated visitors. Any user logged in, with any capabilities, can see the site.

Smart Content Restriction & Membership Control plugin for WordPress. Restrict pages, posts, or custom content by membership level, login, or membership — lightweight, fast, and beautifully simple.