The black buns are sandwiched between mayonnaise and vegetables such as "wapper" size (about 113g (3.99oz)) beef patty, pickles, tomatoes, onions, lettuce and paprika, which means "extremely large", and tomato ketchup with squid ink and garlic. It is said that it is seasoned with BKJ's original "black ketchup" which is a blend of such ingredients.
By the way, Burger King Japan is celebrating its 5th anniversary this year since it opened its first BK store in 2007. This "Black Burger" is a product commemorating this, and was planned with the theme of "insane" in order to aim for a "royal road, but a heretical new leader position" that overturns the concept of the fast food industry so far. That's right.
I sympathized with the part of "a heretical new leader position", so I'm thinking of supporting it a little.