I don't think he's saying it won't flower under only blue. The way I have understood his argument is that if you could get a light that only emitted light in shorter wavelengths than ~620nm (basically cut out all the red spectrum), and left it on your plant 24/7, the plant would in fact think it's dark all the time, and as such would enter a flowering phase. So basically he is saying that as far as night cycle regulating, red is the only color of light which the plant 'sees as light'. You can take a look yourself at the origins of his question at
https://www.rollitup.org/advanced-marijuana-cultivation/354520-green-light-spectrum.html.
For the record, I don't agree with him. It's possible the black light just isn't putting out enough energy to interrupt flowering, at least in usable bands, it started to flower and is committed to finishing, or there is some auto-flower genes in there that he's not aware of. His extrapolating his experience with the black light to saying that only red light regulates flowering seems very unfounded. Just from the way he talks I can guarantee I have done way more research on the topic of light spectrum and photosynthesis, and have yet to see him give one shred of scientific evidence, or a source, which corroborates what he is claiming.