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I found the same dark gray fluid when I changed at 1500 miles. There is another thread where someone pointed out that the black color is coming from the carbon black coating on the diff gears when they are produced (and before incorporated into the diff housing). Supposedly the black coating would come off fully after 2-3 diff fluid changes, and subsequent changes would see fluid that looks much more like fresh fluid.
Since your experience seems to differ a bit (plenty of black color in 3rd fluid change), I'd guess it might be due to how you use the car (road use only, no track). Perhaps using the diff more aggressively (twisty track with aggressive throttle) will get the rest of the black coating out of there for good.
Not sure about the metal particles though....
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