I think it comes out of the economic bubble era of the late 80s, when Japan was obsessed with finding extra ostentatious luxuries for the "new rich" to frivolously spend their wealth on. Up till then "A class" or "grade A" had been the highest possible, but then they introduced "S class" to be like, "This is even MORE high class and MORE expensive!"
They also had a "black" credit card which was one cut above platinum. According to rumor, if you had a black card you could do things like have stores to open in the middle of the night just by calling your credit card company. Like, if you wanted to buy shoes at 3 AM, you just call the credit card company, give your demand and location, then they would tell you to hang up the phone and wait. A few minutes later, they call you back saying that such and such store at such and such address will be open just for you, so please go there.