Turns out character tendency carries over after you finish the game so I killed 4 of the black phantoms and just went on to new game+ since I need a bunch of the boss souls again anyway for the all spells/all miracles trophies. That leaves me needing (I think) one more black phantom kill for pure white character tendency.
I got the trophies for assisting with a boss kill and invading last night, dropped my sign at the flamelurker gate and got summoned within seconds several times. One guy summoned me twice and both times he ran straight into the first flamelurker, started hacking away and was dead in seconds. I do wonder how he even made it that far.