To store file uploads on backblaze, you will need to register for backblaze account, create a new bucket and enter credentials for it into your site "admin > settings > uploading" page.
Signup for backblaze here.
Sign in and click on create a bucket button.
In the new window that opens, verify your email.
After your email is verified, go back to bucket creation. Enter any name and leave the other options at default (bucket private, encryption and object lock disabled) and click "Create bucket".
Note the bucket name and region values, you will need to enter these letter in the settings page on your site.
Click on "Application keys" and "Add new application key". In the window that opens, enter any name for the key, leave all the options at default, and click "Create new key" button
Select All for Allow access to Bucket(s) field, otherwise direct upload functionality will not work.
Copy and paste the "keyID" and "applicationKey" to the appropriate fields in "admin > settings > uploading" page on your site, along with bucket name and region.