Following recent forum discussions etc, it apears to be clear cut that there are potential legal issues with "blue face" feedbacks, and also, that there is no really way to prevent "revenge" feedbacks.
As such, is there really much value in the system of blue faces at all? A potential problem with the current situation is that perhaps that in some cases "blue faces" probably should have been left, but the person has avoided doing so for various reasons- meaning it would appear that someone is "100% positive" when in fact not all people they have worked with have actually had a positive experience on the shoot.
Perhaps keep the system of mentioning of a person no-showed or cancelled late, but simply as a note on the references. (IE, a neutral)