In his book he demonstrates how this can be so. Two identical images are printed, one using the blue filter and the other without. The difference although slight is clearly visible.
When I first started printing I got the Gene Nocon book and for the life of me I couldn't see any difference in the two images but put this down to the standard of book reproduction. Then I bought Tim Rudman's book and noted that he was sceptical. I then read around further and looked at comments on forums such as these and must admit that my conclusion is that the jury is still out. If it really was that clear a difference then you'd expect the weight of opinion to favour the blue filter and so far I haven't seen such a weight of opinion
I remain agnostic