I'm not 100% sure on this, but I believe the way that blacklists work is when there is a blacklisted account that has logged into your ip, all accounts that have logged into that ip will be unable to play.
Let me make an example: lets say we have 3 accounts for 3 players.
Player 1 is blacklisted
Player 1 shared his account with player 2.
Player 2 shared his account with player 3.
Player 2's account will be blacklisted in relation to player 1
Player 3 will not be blacklisted.
Hopefully this isn't too confusing