What is Black List?
Black List is a World of Warcraft addon that adds a new players list (called "Black") to your Friends Tab on the Social Panel, near Friend List, Ignore List and Mute List. It adds a "Black List" tab to the friends frame. This is like Ignore, except unlimited amounts of players with editable descriptions, class, race, level, faction, and detailed reason for blacklisting them. Optionally automatically warns you with sound and a screen message when a blacklisted person is moused over, in a group or raid with you, or in your guild. This mod can even auto kick blacklisted players from your guild if they sneak in (and you have permission to do so). Also optionally rejects whispers and party invites from blacklisted players.
How I use it?
It's easy to add someone to your Black List! You can:

right click con the Target frame and shift click on "Add to BL" menu;
click the "Add Player" button on "BL" tab in Friends Tab on the Social panel;
type /blacklist (or /bl) into your chat box.

For the slash command, you may also specific the reason with the name, "/blacklist <Name> <Reason>".
If you have someone targeted, Black List will add them and populate the details automatically. If you mouse over him, Black List automatically update information for that player.
"Warn me" checkbox will toggle warning you with text if the player in list name appears with /who command or in Who tab in Social Panel, or you receive a whisper or a party invite from the person be in the same party as them; it also warns you with text and sound if you target them or mouse over them.
You can also edit their level, class, and race by clicking the "Edit" button on the details popup or edit the reason by clicking on the textbox and typing.
To remove someone, click their name in the Black List and click the "Remove Player" button.

Why should I use it?
For people of the same faction, think about ninjas (for example people that need objects useless for them), or people that use blasphemies insted of commas, or who leaves a party without reason, or "bad player", etc.
For people of the other faction, think about people attacking you while you're in combat, or camping you, or attacking you only when they have a lvl 70 friend near them, or "/rasp" and "/spit" you in neutral cities... do you need other examples? :)
Compared to Ignore List, Black List can be very useful because you can add a reason to a player, and it remembers the names and the "why" about people for you. ;) Black List has been abandoned by the original author.
The torch has been passed on to ElrickEnonimis as author of Black List, as i sure like to add who wants to continue the development of this addon.