Hi,
There is also another case where a blacklist could be useful.
When there are thousands of model attributes a blacklist is more quick than a whitelist :
This could be very time saving.
one dirty example:
if ($model->validate('*',array('except'=>'title')))
My another suggestion for a blacklist was @ model rules:
public function rules() {
return array(
array('title','required','skip'=>'at_a_specified_scenario'),
);