"Black List" is a list of blocked email addresses - email addresses you don't wish to receive emails from.
If you add an email address to the Black List, emails coming from this address will be treated according to "Email: Black/White list options" settings (either deleted immediately or kept in the Spam folder or kept in the "Spam" folder and marked as read.
If you select the "Automatically add/remove sender's email address to/from blacklist when moving message to/from Spam folder." checkbox in the "Automated Actions" section of the "General Settings" tab, then anytime you move an email from your Crawler to the Spam folder, the email address of its sender will be automatically added to the Black List. Likewise, when you remove an email from the Spam Folder, email address of its sender will be removed from the Black List as well.