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The effects of ultra-sonic action alone should not affect the color of metals except under extreme circumstances.
However the chemicals used in the cleaning process can affect texture and surface of metals depending on the relationship between the metal and the cleaning fluid.
The blue hue is an oxide of iron and if the cleaning chemical will reduce or react with oxides of iron, then there will be some change depending on temperature, strength of the chemical and exposure time.
Normally, petroleum solvents have no reactive effect on steel parts regardless of color.
All that being said, the simple functional test for any ultrasonic cleaning device involves immersion of a scrap of aluminum foil dipped into water agitated ultrasonically. The appearance of the foil will be drastically affected!
Why do you ask? Have you encountered a problem with the cleaning process?
I'v just never put blued parts in it before, only clock parts. My son brought over a gun to see if I could clean it in my machine and I didn't want to damage it. Wanted to be sure. I did take and old gun part and tried it and as you said it had no effect on the finish but did do an excellant job cleaning it.