How safe is my email address?

You should read the privacy policy of Automattic.com

When you leave a comment on a blog at wordpress.com and if you are logged in your email address is made available to the owner of the blog when they see your comment. This is the same as you see when people comment on your blog.

If you are uncomfortable with that, you must log out of wordpress.com before leaving a comment. Even then you will have to leave an email address but it will be your choice as to what you enter. Some blogs will require you to be logged in to make a comment - in that case you cannot.

If you leave a comment and realise you were logged in, do not send in a feedback / contact support. Nothing can be done.