The black list on this site exit as a means to check to see if an address is blacklisted.
These blacklists are typically used by mail server to check against server that have been abusing
the internet in some way. Most of the black lists typically operate an easy on an and easy off policy.
This site provides a means to check to see if an ip address is listed on some of the more common black lists.
If oyu are intrested in finding out how DNS Black lists work try http://en.wikipedia.org/wiki/DNSBL