Hi Ed,
Yes, I set it to black temporarily to test. If I set the body background to pure white, only the transition for the header area looks odd as it starts its transition later than the rest of the page.
When I set both to for instance #ccc, not only the transition looks odd but also the end result, because the color for the header is different than the color for the rest.
If I make the overlay background different from the body color it gets worse.
I think there is a difference in the way the transition is done for the two areas and the opacity of the end results seems different.
I have now set both to #ccc: http://nio.photography/about-buying-prints-licences/
Cheers,
johan