Using the "Locate" command I located the catalog file that I had copied over from the old server. In this case there was one called "retro-misc-ny-B2.rbc", and a folder called "retro-misc-ny-B2" containing files like "FileServer-macsrv001_Users_admin_Desktop.rbc", "FileServer-VMFS03_Helios.rbc", etc. Using the locate command on any of the ones in the folder does not return an error, but does not seem to do anything. Maybe I'm wrong but I don't think the "locate" command is meant to work with the actual backups - for that I would have to do a rebuild, which would take a very long time I think. (Backblaze is slow, but also relatively inexpensive as compared with Amazon for example). I'm not sure what was meant by the suggestion to "add" - in Media sets there is the big Add button which would simply add a new media set - not an existing one as far as I know. Then in the member list there is a plus button which would add another member, but that would also be a new one, not an existing one.
The locate command worked insofar as I can access the full media set of backups and can restore files from it. The problem is that, from the point of view of this new server the sources are different, and therefore all new backups are going into new container folders in the cloud media set, so there is no de-duplication. But at least I have access to the older backups, which is nice.
The listings indicating 0 of 4 TB space used are clearly in error, because all the backups are there and can be accessed.