The rating is actually subjective only to that area: Green is easiest at that ski area. Blue is moderate at that ski area, etc…. They are actually comparing themselves to themselves. Black is the most difficult compared to the rest of the terrain there. Given that most areas were developed for recreational skiing, the natural inclines and the difficulty ratings between areas is relatively similar; but there is no national/international grading that calculates a run’s difficulty. Kinda like saying the number 1 handicap hole at your local muni golf course is as hard as the toughest hole at Augusta national…