You can download the whole book from Ctein's site for free and read that section if you like. It may provide a more complete explanation.
I also remember someone posting here that they did test it and found just the opposite, that is, they had sharper prints using the blue filter. I believe they also had a blind test from 3rd parties choose the sharpest print, which was the one printed with the blue filter focus. Personally, I think it works for some people and not others due to individual ability to focus blue light with the naked eye.