Wow! Two pages and as yet no-one seems to be saying that you should always focus at the aperture at which you intend to expose the paper. I seem to recall that in the past when this question was asked there were those who insisted that there was always focus shift from wide open to a smaller aperture irrespective of how good the lens was

Maybe we have lost those members who believe that

I must try BMbikerider's suggestion on simulating a blue "look" to see if there is any difference. I had read the same G Nocon book and while he uses a set of "before and after" examples in that book I must confess to not being able to see any difference. Gene did concede that in the book's limited reproduction range it might not be apparent and it certainly wasn't apparent to me.

No less a printer than Tim Rudman in his book said, if I recall correctly and if I understood his meaning, that he remained unconvinced of the blue filter's value