Thanks for the kind comments Ruediger.
Yes, I dried these prints face down on a screen to try to control curl.
Low relative humidity this time of the year here has a lot of FB prints going curly on me presently unless dried face down.
I thought that I had washed the screens well after doing a cyanotype coating run.
Ocassionally some sensitiser gets on the back of the prints since I coat with a rod.
Hence some can get onto the drying screens when the sheets are laid out to dry there, and why I wash my screens.
That I suspect is where the brown comese from.
The ferri-ferro prussian blue dye in the dark looks to have broken down to plain old iron oxide that worked its way into the soft wet gelatin face of the print.
Sorry I did not notice this before the print got out in the mail.