Yeah, I expect that's just someone with a mania for Wiki-standardization. It's not a precise shade; any cyan-ish color would do. In practice non-repro pencils and markers varied from sky blue to a rich turquoise.

The article seems confused - it's implying that there is some magic shade of blue that cameras can't see (even today), which is totally wrong. I think that's why someone found it interesting to post here.

Graphic arts film wasn't at all fussy about the shade of blue (as you note) and so while there were expensive non-repro blue markers and pencils, everyone I knew (at the very end of the era of graphic arts cameras) used blue highlighters, so design studios were full of them.

I've stuck with blue as the only highlighter colour I'll ever use, more than 20 years since the original rationale.

Also, used to freak people out scribbling (non-repro) obscenities on a flat that was going to be sent to photo and turned into a newspaper the next day.

Especially since an sRGB triplet only specifies how to perceptually reproduce the color, and film has a different spectral response from the human eye. The dye in non-photo blue probably should actually spectrally be in the blue range rather than having any dye in the red or green range, since it would likely show up on film otherwise.