Hi,
since a few weeks Backblaze added support for Application Keys limited to a single bucket.
Using those keys the plugin does not work because you have to use the KeyID and the ApplicationKey to login, but then you need to use the AccountId to make requests such as listBuckets.
I think the best solution is to use the provided AccountID and Application Key for the b2_authorize_account request, then save the AccountID returned by that call along with the AuthorizationToken for the subsequent requests.
https://www.backblaze.com/b2/docs/b2_authorize_account.html
https://www.backblaze.com/b2/docs/application_keys.html
Thank you