iframe shows as blank page in Confluence

If your added iframe macro shows as a blank page in confluence page. I think there are two places you need to check.

1. Ensure the site url (include protocol scheme) has been whitelisted.
2. Check whether the site allows cross origin access.

For example, *.sharepoint.com does not cross origin access by setting x-frame-option as SAMEORIGIN.

Reference:
https://confluence.atlassian.com/confkb/how-to-put-an-iframe-into-confluence-598837531.html
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options