The primary project of the “Blitzed” group is the Blitzed Internet Relay Chat (IRC) network.
They also operated a DNSBL zone called opm.blitzed.org. This was the Blitzed Open Proxy Monitor (OPM). This popular open proxy DNSBL was run in such a way as to not probe a remote server to determine its open proxy status unless the server was implicated in reports of abuse. It did not list open relays.
The Blitzed group seems to have suffered a database or server failure as of May, 2006. This email to the “OPM Announce” mailing list details the situation, and explains that the OPM list would not be resurrected.
The list is not active at this time.
Based on this information, I would recommend that you remove opm.blitzed.org from the list of DNSBLs being checked in your mail server. It will no longer block any spam, and the potential exists for unpredictable results to be returned. Additionally, you'll be generating unnecessary DNS query traffic to the Blitzed network.