Nilay from Backblaze here. Amongst other things, the solution engineering team reports up to me... and when I saw this comment yesterday, I had them dig into why the B2 S3 API doesn't did not work for you.

It turns out that Backblaze B2 absolutely does support uploading objects via S3 style presigned URLs. However, there are a couple of caveats:

1. B2 doesn't currently support PostObject operations. Therefore, you must use PutObject to upload to a pre-signed URL. Many of the AWS SDKs default to PostObject.

2. B2 only supports AWS v4 authentication and not v2, which AWS has deprecated. However, some AWS SDKs default to v2.

I've put together a very simple python code sample that successfully works for me. https://gist.github.com/nilayp/2c2a04f033d8992ce4b8f591ab449...

Would this solve your issue?