Very Normal. It has to do with sunlight reflecting off them . Also over time their beaks may fade to a blue- totally normal as well..and further down the road- if female they will start to show white feathers growing...just a sign of age. Its a pity that happens so often in the black ducks as they are absolutely gorgeous.
But you..
You may want to put up some pictures of those duckies just to be sure...
My Black Runner, Raven, has that beautiful green sheen in the sun, a beautiful all black bill and lays beautiful green/blue eggs.
I wish you could see her better in this pic. See her in the group? I really need to take some new ones. This was when I first got them a few months back and they were in their little newbie pen waiting to be introduced to the flock..