Hi Ferry,
The citric acid will darken the edge considerably if it's tempered. If you have the differential temper, then it may mean you have a forged Japanese sword. If there is a hamon, then Mr. Mishina may be more interested to examine the sword. If there is no temper line, then it could be the mass-produced WWII kind or a newer piece.
Here is an example from one of my blades.
Emanuel
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