That’s exactly correct, the Blackhole trigger is the query string portion of the URL. For example:
https://example.com/?blackhole=1234567890
Any bots that disobey the robots.txt rule and follow the URL will be blocked from your site. So it shouldn’t matter if you exclude the URL or query string in your CDN, cache plugin, etc. In fact, it is recommended that you do so, in order to prevent the Blackhole from blocking your server IP.
To test proper functionality after adding the query string to your CDN, follow these steps:
1) Visit your site via any proxy service (there are many available online)
2) View the source code of any of your site’s web pages
3) Click on the Blackhole link
4) Check the Blackhole Log
After following these steps, you should see the IP address of the proxy service listed in the Blackhole Log.
I hope this helps, let me know if I can provide any further infos.