The delay starts counting when he user is created so that might have confused you.
If you do not want the screen darkening feature to stay why do we need an option to suppress it? I’m a little confused by the use case and the request here. 
@tobiaseigen May I get the reproduction steps for this? I just tried locally and I do not get a black overlay. The black overlay only shows when there is an unread PM and the user has not read any notifications.