There isn't any issues with the InstaMail feature that I am aware of. But I can check into it for you. I'll probably get an answer by middle of Monday as the programmers aren't around to ask right now.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .