I left some postwar truck sides soaking in evaporust overnight instead checking it every few hours and yes that did remove all the chemical blacking and turned it whitish. I returned it to a good appearance by blueing it.
How did you blue it? Which product and what process? I looked thru the spare trucks I have and found a candidate to experiment on - half of one side frame is "normal" while the other half is missing all chemical darkening - This truck is useless as-is, so it would be a great one to experiment on. Plus I have several other trucks where the black appearance could use improvement.