The reason for that is a black diamond is more rare and people love it more(for some strange reason)
because black diamonds are just bad diamonds with a crap load of inclusions the more clear the diamond is the more perfections it has
The price of the diamond depends on the 4C’s i.e clarity, carat, color and cut. The diamond that is colorless are the most costliest bcoz they are the natural ones. Colored diamonds have more impurities than colorless. So black diamonds are less costlier than regular diamonds.