BlackIce Defender 2.1 and earlier, and BlackIce Pro 2.0.23 and earlier, do not properly block Back Orifice traffic when the security setting is Nervous or lower.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2000-0558
http://archives.neohapsis.com/archives/bugtraq/2000-06/0190.html