WHY DO THEY CALL THIS AREA THE BLACK HILLS?

There are no really high mountain close around but there are some high hills.  Every one of those hills from the lowest to the highest are covered with Ponderosa Pines.  Close up, the pines are pretty and green with some aspens, maples, box elder and whatever else mixed in.  From a distance, however, all the pines look much darker.  In fact, the hills in the distance look black.