For using Backblaze B2 as a special remote, there are currently three choices:

• Using rclone
  (Actively maintained)

• Backblaze B2 supports supports the same API as Amazon S3, so git-annex's built-in S3 special remote can be used with it.

  However, it needs S3 version 4 signatures, which are only supported by git-annex 8.20200508 and newer.

  Here is how to set up the special remote:

    git annex initremote backblaze type=S3 signature=v4 host=$endpoint bucket=$bucketid protocol=https
  

  Remember to replace $endpoint with the actual backblaze endpoint and $bucketid with the bucketid.

• A dedicated special remote, https://github.com/encryptio/git-annex-remote-b2
  (Last updated 2016)

At this time it's not clear which is better, so if you find one works better than the other, please comment below.