Aha!  Courtesy of Dave Filcoff, we have a first hand account of why
     alloy lug nuts that many of us believe to be (have been) chrome now
     appear to be black.  Could it be?

          The front lug nuts on my van (alloy wheels) were silver when it
          was new, they have since turned black, so I used a Testors Chrome
          Silver model paint pen on them, they look just like the rear
          (chrome) lug nuts.  The only problem I can foresee is chipping
          the paint when I need to take the wheel off, but I can just touch
          up the paint.

          Dave Filcoff

     -Blaine