I have found online that if you use coffee and tannic acid, then washed in Soduim carbonate.

Here is the image, its very close to black and white:


Taken from the description below the image:

Coffee+Tannic Acid wash
Sodium Carbonate wash
(Repeat Coffee+Tannic acid wash , Sodium Carbonate Wash)
Rinse in cold Water
I had thought this cyanotype was ruined when I did the first Sodium carbonate wash - the image developed ugly brown spots.
Thankfully I finished processing anyway. The spots faded once the paper was dry - Whew!