Image taken from Burger King Japan's press release on their web site.
The burger's bun is black because it has been mixed with squid ink. I'm sure that'll get your mouths watering. There is also something about bamboo charcoal being used to bring out the flavor of the burger and the bun. Some outlets are mistakenly reporting that the buns are black because of the charcoal, but I think that's a mistake as they'd have to be burned to a crisp to be so black due to any sort of cooking. There's also paprika as part of the flavor profile.
If I were still in Japan, I wouldn't touch this with a 10-foot pole because I don't eat beef. My husband probably would have tried it, but I'm guessing even he might have been put off by the notion of squid ink in his food. I'm sure that it's far less disgusting than it sounds, but it sounds fairly gross. Anyway, by the time you read this, the burger will be finished since it'll only be available in limited quantities for one day.
Image taken from McDonald's web site.
On the side of BK's rival, McDonald's, there is a somewhat more compelling item. They're selling a "MacFlurry" which is blended with strawberry sauce and little bits of KitKat. I don't know if it is a coincidence, but this is being released on the same day as the black burger (September 28). It's available for 230 yen ($3.07) from 10:30 am and can't be had at all shops, but it will be around for longer than a single day.
The image makes it appear that the bits of KitKat are actually pretty tiny. This isn't surprising since too much candy in it would make the result too sweet for Japanese tastes. The big focus is on the strawberry sauce. If anyone in Japan gives this a try, I'd be curious about the sweetness level and the textural aspects. It seems like very little of the wafers would come through with such specks of KitKat as shown here.