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Left is a normal coloured 20k. stamp, in the middle the variation. The variation stamp is more like the 10k. stamp from the same set to the right.

Something chemical? The sun? Or am I a millionaire??

Is there a gum?
Looks like someone tried to remove foxing with Clorox bleach.

Yes. The stamp is gummed. The gum looks typically for that period, and the stamp is MLH. Both circumstances advocates that it is original gum, but of course, you can never be 100% shore.

Well, I know that it's possible to transform the green color into different shades of blue using different chemicals. Though, I don't know the exact formula.
May be, in your case it was the sun, indeed.

There is no information in catalogs on this stamp in a different color.
Proofs had the same color, but different perforation.

Green ink is often an mixture of blue and yellow ink. It doesn't take much sunlight to start the yellow fading, leaving you with a blue stamp. No chemicals required.

_________________
Greg - Looking for Goulburn Australia Cancels and Grangemouth Scotland Cancels and Covers
Member of the S.T.A.M.P Club for Slightly Twisted And Mad Philatelists - Motto: "Bring back the lick!"

UV exposure makes paper also yellow and brittle faster than someone would expect. And the resulting blue shade would not look very deep or even.

Greg,

That's "no chemicals" if you discount the billions of tons of burning hydrogen, I guess