When you register a username account all that's required is an email address and it becomes your unique identifier. http://en.support.wordpress.com/email-address/ You cannot access any blog unless you are logged under the exact same username account that registered the blog.
You have two separate email address settings at WordPress.com.
Blog notifications for the admin (like comment moderation, Likes, new subscriptions, etc) are sent to the email address at Settings -> General in the Dashboard.
Personal notifications (like comments on your post, subscription emails, and upgrade renewals) are sent to the email address at Users -> Personal Settings in the Dashboard.
Try changing it at http://wordpress.com/#!/settings/ first.
If that doesn't work, try Users -> Personal Settings in your blog's Dashboard.
Note: Issues Changing your E-mail Address > http://en.support.wordpress.com/email-address/#issues-changing-your-e-mail-address