I believe that the result for an unblocked mailhost on a valid DNSBL service is the same as the result for any host on a non-existent DNSBL service.
That is, the following two commands generate the same results:
nslookup 105.3.12.83.combined.njabl.org
nslookup 107.3.12.83.combined.njabl.omg
Compared to this query, which returns a positive results (as of this writing):
nslookup 107.3.12.83.combined.njabl.omg
I believe that DNSBL sends a query simultaneously to all of the configured DNSBL servers, then waits for a reply. The lack of a positive reply in the timeout period (which looks like 30 seconds, from the contents of /usr/share/qpsmtpd/plugins/dnsbl) causes dnsbl to reply 'DECLINE', which allows the message process to continue.
http://rbls.org can be used to show you the results from many different rbls for any IP or domain name.