Well Vin says that there's something behind all this, right? Some evil force of doom or whatever? Well, if I were said force of doom, then I certainly wouldn't have used my powers to turn the land black. It just lacks flair. Red. Now, that would be an interesting color. Think of the possibilities--if the ash were red, the rivers would run like blood. Black is so monotonous that you can forget about it, but red--you'd always be thinking, 'Why, look at that. That hill is red. That evil force of doom trying to destroy me certainly has style.
Brandon Sanderson (2011). “Mistborn Trilogy”, p.1499, Macmillan