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