At least custom domains are now natively supported.
Custom Domains, launched today, lets you serve content to your end users from the web domain or URL of your choosing, with no need for complex code or proxy servers. Backblaze manages the heavy lifting of cloud storage on the back end.
Custom Domains functionality combines CNAME and Backblaze B2 Object Storage, enabling the use of your preferred domain name in your files’ web domain or URLs instead of using the domain name that Backblaze automatically assigns.