(http://oyster.ignimgs.com/wordpress/stg.ign.com/2014/09/10700756_822395754491427_29764032235952239_o-720x540.jpg)
They were apparently first available for a limited time back in 2012 and were popular enough to bring back the following year and now again. So it has become somewhat of a yearly tradition in Japan.
The bun and cheese are made black with bamboo charcoal, and the ketchup is turned into a sauce with squid ink.
Two types are available, the Kuro Pearl and Kuro Diamond. "Kuro" means black in Japanese.
The Pearl costs about $4.50 and is a plain black cheeseburger (with squid ink ketchup). The Diamond adds onion, tomato, lettuce, and sauce for about $6.50.
The Kuro burger is only available for a limited time.
From IGN (http://www.ign.com/articles/2014/09/12/black-burgers-back-in-japan)
(http://i.dailymail.co.uk/i/pix/2014/09/11/1410438303238_wps_52_Capture_JPG.jpg)
(http://i.dailymail.co.uk/i/pix/2014/09/11/1410437155196_wps_51_Burger_King_Japan_s_new_b.jpg)