The e-mail address registered with your WordPress.com account is your unique identifier. You will notice that when you sign up, we do not require any other information. As such, this is how we are able to verify ownership of blogs and accounts; it is very important to remember and update it as necessary. http://en.support.wordpress.com/email-address/
Note: 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.
Change here first > http://wordpress.com/#!/settings/
See > Issues Changing your E-mail Address > http://en.support.wordpress.com/email-address/#issues-changing-your-e-mail-address
In particular, I'm wondering how this might affect my access to private blogs to which I've been invited and made an author. If I change either of these, might I lose access to the blogs in which I'm an author?
I don't know so I'm tagging this thread for a Staff response. Please subscribe to the thread so you are notified when they respond and please be patient while waiting.