Typically, a blog is for you to share - with your friends, or with one special friend.
It doesn't have to be though. It's just as normal to have a blog that only you can read.
If you have a Public blog, and you want to close it down temporarily, you can do just that, with very little effort.
Just Edit Blog Readers, in the Permissions wizard, change to "Private - Only these readers", remove any existing readers, and don't click on "Add readers". When you want to re open the blog, change the Reader list to "Public", "Private - Only blog authors", or add your readers back (if necessary).
While you close the blog, if you have other administrators, and want to keep them out, you may want to suspend their administrator status.
Of course, this won't make any blog completely invisible, on the Internet. All of the search engines - Google, MSN, Yahoo, and the others - will have the blog content in their cache, and their SERPs. That cache won't go away immediately - or at all, in some cases. This will simply make access to the blog details, whether from a feed or a search engine, unavailable.