As stated, the potential abuse of some kind of blacklist is way to big to delve into. You'd be better off cultivating relationships with folks from around the game, and garnering a kind of referral network. If you have a couple of people you've associated with well enough to trust if they give you a disclaimer about a potential recruit, you can make a better informed decision. But as with anything in life, always take everything you hear with a grain of salt; unless said person has a known history of repeated offences, it could just have been an isolated case of one or either party handling a situation badly.
There is also a matter of matching players and fellowships with the right fit; that is a very imperfect process. As stated, if an Archmage is sore because a member left for another fellowship, you have to keep in mind there may have been valid reasons for the player for doing so. It could have been anything as simple as finding a better match for their playstyle, or something more complicated like a leadership disagreement. It would be very improper to share some examples, so I'll just tell you that 1: recruiting is a pain and you'll get a lot of players that aren't what you are looking for along with the ones that you want, and 2: players will come and players will go. So if you have folks who leave to join a fellowship better suited to their needs, wish them well and move on.