That could work well because if the message shows the domain that looks like the content they want to see they will allow it but if its a random tracker domain they will block it.
It's also important to not include [Whitelist INC i.imgur.com], only [Whitelist INC i.imgur.com FROM example.com], because trackers will adapt to exploit the first one as soon as it's common enough to be noteworthy.
If browsers break 3rd party cookies and they are not fingerprintable, the arms race stops: there is no way to track a client across multiple sites, other than weak correlations with IP, interests, etc.
