Ok, just in case anyone else is struggling to get 2.3.7 working with Backblaze B2, I finally got things working after hours of trying. I've attached the config that works with 2.3.7. Remember to replace all of the placeholders with your correct info in the file.
Something to note - it takes Backblaze a lot of time to actually show the files in the bucket. I initially thought it wasn't working because I didn't immediately see the files, but it sometimes takes about 30 mins for them to show in bucket dashboard.