Ok, I figured this out.
I needed to configure the DNS forwarder to be authoritative for the blah.com domain.
Also, on the same setup screen, I needed to set the local IP for server.blah.com.
Now, I can use the fqdn if I am at the home office or on the road.
I LOVE pfsense !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!