I may be misunderstanding what you want to do, but have you considered
using an iron blue toner on the silver print? The result is a screaming
blue color that is equivalent to the blue cyanotype image.
Edwal and Berg both market iron blue toners, though I've always preferred
to mix my own. Blue toner formulas are as numerous and varied as
photographers. For the sake of interest, one which has always given
me good results is:
Water (110 degrees F) 500 ml
Ferric ammonium citrate 4.0 grams
Oxalic acid 4.0 grams
Potassium ferricyanide 4.0 grams
Cold water to make 1 liter
It is true that the action of the pot. ferricyanide will bleach the
image a certain amount, so you should plan to print any image you
wish to tone in this formula a bit darker than you might normally.
If on the other hand, you wish to print a separate image using a
cyanotype formula over a silver print, then I'm stumped.
Hope this helps,
Carson Graves
carson@ileaf.com