I've used vinegar to strip blue off of gun parts.
Don't try this at home for sure!
Stripping out the carpet and painting the insides with Kilz Primer might be the best solution.

....and doubling up the GoldenRods, and installing Hygrometers in each one...

Will Lemon Oil hurt the bluing? I suspect it has a neutral pH...

Hot lockers
 

In Vietnam during the monsoon seasons we would install a porcelain keyless fixture with a 60 watt bulb in it to heat the insides of our lockers and keep clothes and firearms dry. It generates a respectable amount of warmth and keeps things dry. Use a 130 volt bulb for much longer (bulb) life. You could probably get away with a 40 watt.
I agree with Daryl that the carpet is holding humidity. Get rid of the carpet in one safe and try the hot locker treatment.

I found a supply of 15W 130 volt bulbs, so a couple weeks ago we installed porcelain fixtures in the safes above the center partitions, nowhere near the stock wood. They seem warmer whenever you open them, so maybe this will work. The owner wouldn't let me rip out the carpet. I won't name names but the safes were sold recently by the successor to Parker Bros... Thanks again for all the suggestions!