I think it would depend on the circumstances - including the reason why she's on the blacklist. if a WG gives good service to some punters and bad service to others, then she should get a mixture of good and bad reviews - but she shouldn't end up on the blacklist just for giving a poor service, it means that something more serious has happened, and in some cases she might actually be dangerous.
Even if she gave you the best sex you've ever had, that doesn't mean the other guy wasn't threatened / attacked / robbed / blackmailed / outed or whatever. That subject is going to come up, so any attempt to boost the reputation of a blacklisted WG is likely to backfire.