My initial reaction to the original question was like others, "they're all blue". But photos to the contrary tell a different story. I just looked up some photos I took of C501 arriving in Melbourne on its delivery run, and as in duttonbay's photo above, the frame is black. The covering on the long duct on top of the frame on the left hand side though was blue.
Going back to the question, a valance
is defined as a decorative cover, and I think that's where the distinction seemed to have been made on VR diesels. The loco frame
(not decorative) on hood units was black, but "decorative covers" such as the duct cover on the C, the covers above the fuel tanks on the B and S classes, and the deep valances on 1st and 2nd series Ts (later cut away to just the frame) were all blue I believe.
So on that definition, I'd venture to say that valances, as distinct from frames, on VR diesels were blue.