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Yes, I believe you answered your own question.

Although, you can do more rudimentary blacklists in the firewall aliases. 

Wow, this was maddening

I finally figure out what was happening.  The XMPP server is normally determined using the SRV record from DNS.  https://kingant.net/check_xmpp_dns/

Our AT&T uverse ISP (some sort of DSL I think) is stripping SRV records from DNS queries.  On our old PFsense setup the DNS servers from the other ISP were given the highest rank.  On this release of OPNsense the DNS entry determined by DHCP on the DSL was given the highest rank. 

I can uncheck the "Allow DNS server list to be overridden by DHCP/PPP on WAN " but what if the DNS address changes on that ISP/interface?

Previously, I had an older PFsense release.  Since setting up OPNsense, XMPP chat clients (google talk and another XMPP service provided by our phone company) are no longer able to logon/function.  I do not see corresponding block messages in the firewall log.  I don't understand what is happening. Help appreciated.