Just in case we misunderstood each other. I have executed the minio client inside my minio container deployment. So the network connectivity is exactly the same given that they are inside the same container.

So I was able to add the backblaze backend as an alias site to minio client and then list my buckets content. Even when it was restricted to that particular bucket it behaved as expected. But if I set up the replication via the GUI in minio, it suddenly logs out that the remote bucket is offline. Both URL and access parameters are the same between CLI and GUI

I tried both a general purpose key that had access to all buckets, and one limited to the test bucket. Both times with read-and-write