Summary: Black ducks are certainly my favorite duck decoy to paint, fast and easy! :-) I do want to point out that yes the bill of a black duck is not banana yellow, but we use this color for extra visibility. The only reason we even use black ducks in our spread is for the purpose of extra visibility as we have very few black ducks in Iowa where we are located.
If you want to add a nice touch to your black duck decoys you can take the light tan color used on the head and highlight the tops of the raised feathers on the sides and top of your decoys. To do so use a foam brush dabbed a few times on paper to remove any extra paint, and then very lightly slide it accross the tops of the raised feathers.