The purpose of the blue filter as explained by the late Canadian Photographer Gene Nocon, is, it is a critical grain focussing aid. It is explained in his book about darkroom printing that the light from an enlarger tends to be towards towards the red end of the spectrum and printing paper is sensitive towards the blue end. If the colour of the light from the enlarger is changed to blue, then the image we see via the eyepiece of the grain focusser should be more accurate than without it.

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.

I have a similar focussing aid which was sold in UK under the name of 'Peak' I don't have the blue filter, so I dial in blue light using the colour filters from the colour head, and yes it does seem to work. However stopping down the lens from F2.8 to F8 will cover most inconsistencies, but it is nice to get it accurate before you start to print.

Obviously when printing RA4 the blue filter is not used.