We won't leave your readers behind!
http://myblog.blogspot.com will redirect to your custom domain.
Blogger accomplishes this bit of trickery, using standard Internet protocol - a 301 Redirect - but it's done in two steps.
- When you publish your blog, "myblog.blogspot.com", to your custom domain, "mydomain.com", all internal links within the blog are changed, from "myblog.blogspot.com" to "mydomain.com".
- When the transition period ends 72 hours later, Blogger sets up a standard server based "301 Moved Permanently" for your blog.
- Note that only the blog internal links are changed - any links which you added, from post to post, are your responsibility. Likewise, all external links are your responsibility.
The search engine spiders, which previously indexed your blog as "myblog.blogspot.com", now index it as "mydomain.com". Likewise, your readers see "mydomain.com". To everybody, even those intending to look at the old BlogSpot URL, you now have "mydomain.com" for a blog URL.
The "301 Moved Permanently" referral from BlogSpot is similar to the "301 Moved Permanently" referral used in Custom Domain DNS setups, to equate "mydomain.com" to "www.mydomain.com". In both cases, when this technique is used properly, the clients - either the people who read the blog, or the search engine spiders that index the blog - see the contents of the blog as being part of the redirect target. Now, they see "mydomain.com", instead of "myblog.blogspot.com".
Note that this redirect applies mainly to search engines, following links. It won't apply to feeds, which need the updated URL in their database. It will apply to your readers - but readers who monitor the address window in the browser may become confused, if they see the new, non-BlogSpot URL where they are accustomed to seeing a BlogSpot URL.
You still need to notify your readers, and you still need to update all external fixed URL references to the blog, such as FeedBurner and other custom feeds, Google Webmaster Tools, and the various visitor meters.