I seriously doubt the Panthers are black-balling Cam. If some team was interested enough they would certainly call the Panthers for information, BUT conduct their own physical too. As shown with the Jets, teams are willing to sign a BACKUP QB that's rehabilitating an injury.
Since Cam was being stubborn and insisting on a starting role coming off two injury seasons, I can see why teams stayed away. By the time Cam realized starting wasn't an option, even 99% of the backup spots had been filled. Again, he played his hand wrong in FA.