The black list helps avoid sending duplicate friend requests and automatically contains all the people that are friends or have been sent a friend request from any of your accounts. Be aware that the black list only tracks people per document (e.g. if you send friend requests from document A those people will not appear in the black list for document B).
If you have multiple Facebook accounts you can keep your black list's separate by creating a different document for each account you have. In Flirt you can choose to show one or more accounts on the sidebar.
Choose Flirt > Preferences.
Click the Sidebar button.
Click the Show all accounts checkbox to enable or disable showing all of your accounts on the sidebar.