Hi Serena, I'm pretty sure we've had this community set so that only admins can create private messages (while members can view/reply) forever. Are you asking how to enable private messaging in your own community? (It's a General Permission item you can assign to any Role.)
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 .