Need all users blocked by default

I have a use case where I need all users "blocked" from receiving messages until another user selects to "unblock" them.

Use case:
User A wants to send a PM to User B. (I have it setup where User A will "Flag" User B to notify them that they wish to chat.) User A would at the same time "unblock" User B. When User B gets the "Flag" notification they would then decided if they want to chat with User A. If so, User A would "unblock" User B and send a PM.

In order for this to work all users must be "blocked" at first, by default. In the case above User A & B would be "unblocked" to each other but they will both remain "blocked" [globally] to the rest of the users.

My limited experience with PHP tells me that there is probably a TRUE/FALSE statement in the module that I could just reverse (?). I'm going to continue looking for that, but if there is a better more correct way of doing this I would greatly appreciate it.

Thanks!