Did you try to access your Backblaze bucket using your API key and the minio client? It really seems like either a configuration issue or missing permissions issue. I would also try to create an API key with s3:* permissions to all S3 resources just to be on the safe side.
By the way you can also use AWS cli to do that but you need to define the endpoint, the region and set a profile, so it is a bit more complicated.