I'm setting up Espo for my organization. Posting here just in case someone else is interested in using BackBlaze S3 for upload storage:

1) Setup bucket in Backblaze and a new Application Key for Espo.I think you need to click `Allow List All Bucket Names​` when creating application key.
2) Using instructions in https://docs.espocrm.com/administration/file-storage/ , modify data/config.php​ key 'defaultFileStorage' => 'AwsS3',​
3) Slightly different than instructions for data/config-internal.php​:
PHP Code:
'awsS3Storage' => array(
    'bucketName' => 'YOUR-BUCKET-NAME',
    'path' => '',
    'credentials' => array(
        'key' => 'YOUR keyID',
        'secret' => 'YOUR applicationKey',
    ),
    'region' => null,
    'endpoint' => 'https://YOUREndpointFoundOnBucket',  // for instance mine is https://s3.us-west-000.backblazeb2.com
),​