Can I just say that it is enormously annoying to go to a blog that I've been reading for months or years and find that it suddenly has gone private, without a by-your-leave or anything? And that it would be nice if people who wanted to do this would instead put up a page on the blog that says "Gone private. If you want an invitation, contact me at firstname.lastname@example.org." And then take down the other stuff and start up a new site for your private blog. Because seeing "It doesn't look like you have been invited to read this blog" just hurts my feelings.
Skippy, I'm lookin' at you.