If your computer does support Core Video, then you will see that all
the black levels - except for the pure black and white (its a gamma
curve problem?) - are brighter than they should be.
Did you also compress you test movie to DV and HDV format?
QuickTime displays these formats a little lighter, presumably because
this is better for viewing under daylight viewing conditions.
Perhaps that's what CoreVideo is doing with/to H264?