The trouble is, it did not look so badly blue at the scene. The stage looked rather neutrally lit.
Anyway, would you mind to share your sidecar? RT gave me this as the default:
Sure, although there is a caveat: This one uses the not-as-yet implemented Tone Equalizer. Guessing here that you are computer tech-savvy enough to merge that one.
Here you are: RawTherapee 5.9 dev + #6641
Thanks. A gh pr checkout 6641 should do it, right?
Tricky one. Used the official picture as a reference for spot color correction measurement. Then mostly desaturated blue channel and increased contrast, but the result is by no means perfect.
20230318_0001.RW2.xmp (44.6 KB)
P.S.: Boris, well done on your effort! I learned 90% of my darktable skills from your videos, anyway.
Nice ! I am quite impressed by what you achieved with Color Calibration module.
Probably a matter of taste, but I would add a little bit more saturation and a tiny bit warmer color balance. So based on your edit, here my take :
My interpretation with dt 4.2.1:
Greetings!
This was a real challenge! I feel that since the blue lights were there, there is no need to remove them completely. It’s more about taming them.
darktable 4.2.1:
2023-03-10_19-12-33_P1080015.RW2.xmp (23.9 KB)
Thanks. This looks very strange, though. For example the shadows are much brighter than the illuminated surfaces. Check the area around the man’s trousers, as well as that to the (picture-)right of the lady’s knee, or below where her dress falls from her leg.
I agree! It is the best I have achieved so far after numerous tries.
That’s not surprising, and has nothing to do with your skills: you have a situation with mixed lighting, where different parts of the scene have different illuminants. You cannot “modify” all those illuminants with one white balance setting…
A friend of mine always said “That is the nice thing about colored lights: there is no right way. just make it look how you like it” 
with blues = photography
without = photo illustration
I could go either direction, but to belie the presence of indigo illuminators is deceptive unless you are after an illustration of the event, and not a mere photographic recording. 
That’s a worthy point to consider. However, our eyes and brains get adjusted to the lighting in real life and perhaps wouldn’t perceive it as drastically blue as the camera objectively renders it, so it makes sense to at least even it out a bit.
That’s the reason we usually do white balance, too: The yellow or blue cast you get on a photo with incorrect white balance is really an accurate depiction of reality. It’s just that we don’t perceive it that way in real life, so it strikes us as very odd when we see it in a picture. So we change it to a more neutral representation which looks more natural to us.
In the end it must be the decision of someone who saw the event in the first place what they perceived and what would be a worthy depiction of it.
