The API docs for the create-blueprint call don't give very much detail, and they certainly don't mention anything about how to check the progress of a blueprint snapshot (save/create) operation.
However, when I wrote the save_blueprint command, I determined that after creating a blueprint, I could keep checking the loadingStatus attribute on each vm (<APP>.deployment.vms.<VM>.loadingStatus) until they all showed DONE. This worked great up until about a month ago.
Today I discovered that the app's vm loadingStatus no longer changes at all (from DONE) when you initiate a snapshot (create-blueprint) operation.
When I told Ravello this in a ticket today, they explained that I should instead be checking the vm loadingStatus attributes in the blueprint. Yay.