Agree to this, we need to be able to blacklist people so we don't see them in PF and that includes cross server in the dungeon finder as well. Also, to refute an argument I've heard against the cross server blacklist: it doesn't matter if characters on other servers have the same names as people from other servers. Games that use a cross server block system (like WoW for example) actually factor in the origin server of the person you block and makes sure you're never grouped with them again but not someone with the same name from another server.

One thing to add: Now I don't think this system should extend to someone you blocked joining the same party as you. Only prevent it from showing a party listing from a person you've blocked.