If you wash your blue toned print in water in which you added about 25g/L of
salt (sea salt or cuisine salt) it will give a more brilliant blue. Of course
you will have to wash it again with normal water for more than 10 minutes.
Acetic acid in Judy's formula can be replaced with quite any acid. I tried
with tartaric or very diluted sulfuric acid as well as with hydrochloric acid.
It works. If the Ph is too low then the blue of the print will diffuse in the
white. If the acidity of the toning bath is to low (high Ph) then the toning
will not occur.