Blacklight.. Best White Balance?
The problem with "black light" is that it utilises wavelengths on the edge of human eye sensitivity. Most cameras will struggle rendering it the way we perceive that light (coming out pure blue instead of purple) and it's hard to get that component back in even when shot RAW.
I was always frustrated photographing my kids dancing concerts (and visit to local bowling alley) until I realised where is the problem. Black light has two "spikes" in wavelength graph - the big one is what Bayer sensor can't "see" - it can only see small secondary spike - and "energy" of that spike is very, very small.
To make it simple, I put RGB Bayer sensor wavelength response and black light graph (not in scale, which does not matter) - it make is very obvious that Bayer sensor is limited here as the main spike falls below 400nm, which represents cut-off for RGB sensor.
Hope it helps and answers your question - there is no correct WB for black light.