http:bl is a system offered by the honeypot project[1] that provides
information about malicious IP addresses of visitors of web sites.

To enable the service in ntopng a key is required and has to be specified from
the command line using the '--traffic-filtering httpbl:<api_key>' parameter.

The key can be obtained by signin up to the service [1].

[1] http://www.projecthoneypot.org/httpbl.php