Sorry if necroing is poor form but I ran into exactly OP's issue today while trying to configure uBlock Origin for default-on JS blocking. The good news is that with different configuration it works, and I don't think there's anything that Kagi needs to do here.
There is a long-standing bug 1392090 in Firefox which means that the method of blocking scripts is not allowing <noscript>
to run, and extensions have been opting to either use or not use hacks to work around this.
The problem arises when you use the advanced mode to block scripts globally:
Like OP, I get stuck at the blank white results screen. If I switch on scripts then it comes back to life.
This is kind of okay, but one of the joys of Kagi is that it doesn't need JS to work. Why can't we run without it?
The answer is to use the global "Disable JavaScript" option in uBlock Origin:
With this option checked, there is no longer any need to use the advanced fine-grained filters. Instead, the functionality of the below button in the pop-up is now inverted - sites have JS disabled by default unless you click this to toggle it on.
The only remaining problem is that it seems to take 3 seconds for the meta refresh to kick in. I'm probably going to end up permitting JS regardless. However it is at least not totally broken, so this is going to be the more desirable configuration for someone trying to do this.