well the blue with black ALWAYS looks great. one of my favorite combos... however, i haven't ever seen a blue 370 with gunmetal before... would almost be cool to go that route just to have something a little different...
I'd say gunmetal, because the other cars i've seen this done on seem to always look sharp
i.e:
http://wheelsworldwide.co.uk/img/gallery/2896-l.jpg