When I Follow a blog, I use an Account Profile, which is generally based on an email account. You may use Google Gmail, or Yahoo, or a Blogger Account ID, or even Google's Orkut. Each account will have an email listed to receive copies of all updated posts from the blog.
Nowadays, you really don't even have to be logged in to Follow a blog. If you click 'Follow' from the Navbar, you may never log-in! see: The Real Blogger Status.
The exceptions to this arise from subscriptions to Follower which get copies of updates sent to a news aggregator. Follow automatically sends a copy to Google Reader, which is also linked to your Dashboard, and the Reading List.
So, how would I change the Email Address which is registered, without changing the account profile I use to Follow the blog?
(Need I say it?) You, firstly, have to sign-in to Follower/Friend Connect. Then, click on Options, Site Settings, Messaging. The next image will appear, on which your email address - and the option to change it, will appear.
Click on 'Change email address'
When this screen pop-up appears, it will have your registered email address on it. Simply change it to the one you want, and click 'Submit'.
Naturally, verification of ownership of email, is required...
When verification is received, you'll see this message. BTW, all your subscriptions will be listed as well, as they will be updated with the new email address.