The idea of aging beef works very well when the meat is less marbled and needs to shed water to improve in flavor.

But the gyu in Japan isn’t the kind of beef you can easily age because the meat has so much fat it’s unforgiving. That’s not to say some butchers aren’t trying to dry age, but I cannot see how it helps the beef as the fat cannot age very well, and rarely improves with age. The dark spots are oxidation and when Japanese beef oxidates, it gets sour and unpleasant leaving a wet leather taste in your mouth.