Spam Hound uses information uses online databases of spam senders to better detect spam.

A Blackhole is an online database of computer addresses (IPs).

Computers are added to blackholes for any number of reasons ranging from belonging to a company that continuously and persistently sends out millions of spam, to computers which reside in a particular country.

Blackholes provide very good spam filtering as part of a series of tests like Spam Hound performs especially when configured for an individual user.
For example a result that indicates that the email was sent from a server in China (where security is notoriously lax) is not an indication of spam if you regularly correspond with people in China.
However if you don't, it may be a good indicator that the email is spam.

Spam Hound's Neural Network automatically learns what blackholes are relevant to you.