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.
[Whitelist INC saasapp.com From mydomain.com]
[Blacklist saasapp.com]
[Allow once]
[Ignore]