I tried to get unity using black pigment but it just doesn't  happen. It dulled the background hill by creating a dull pall. Each time I would lighten I would add enough color to give it hue that didn't look black but I couldn't hide it, even in the dark colors. 

Sure it has a lot of contrast and if you only used it in the darker colors you could keep it from turning the lighter colors gray. In the end it was not worth it. The darks do not look natural. 
Painting with black and gray is a different and less complecated way to paint. One doesn't have to think of the color of the shadow because it is automaticly black with another color in it.  The black shows, I'll never do it again.