Adding a php statement

are you sure that your blackhole file is located at the correct place?
try to put another call in the customheader file to see if it works

I added the same line but changed /blackhole/ to /lwHostsCheck.php (as I had that file in my root directory). Interestingly, that line still didn't show up in a Page Source, however it altered the page, changing the color of the background to a light blue (not sure why). But, the fact that a change occurred seems to me that Joomla recognized that a file WAS being called.

I also tried this system on another site and got exactly the same result (not working, but a blank white page). So I'm wondering if there's something in his code that isn't up-to-date with the latest PHP version (I'm on 8.0 or 8.1). I have a contact in to him to see if he has an idea. It's times like this that I really wish I knew PHP!!

if you want to test the call of the file, you can try that this will totally kill your page and only say hello
if it works, then you know that the files are called correctly from the template, then don't forget to remove the die() code lol