I know of no more levels than the ones you describe. There is no blacklist of Apple devices outside of the Find My iPhone service in iCloud.
I would recommend to treat these stories as meaningless unless accompanied by further information. To wit: What does "blocked" mean? Does the phone turn on, but doesn't get phone service? Does switching carriers change things? (Then it could conceivably be a carrier blacklist, which I've also never heard of but at least makes sense in theory). Does it not boot at all?
Normally, if you have the device PIN and the phone is logged out of iCloud, that's all there is to it. Unless there is a detailed and verifiable report otherwise, one can safely assume that any report to the contrary is being fed by ignorance and/or bad intentions.
P.S. I have some new and used iPhones if anyone needs.