In many onsen resorts, you can see “Onsen Tamago” (hot spring eggs) sold.
“Onsen Tamago” is boildegg, made by dipping eggs in very hot water of onsen. Often the condition is soft-boiled that the white is soft and the yolk is hard, which is opposite to standard soft-boild eggs.
These hot spring eggs in the photo are specially called “Kuro Tamago” (black eggs).
“Kuro Tamago” are sold at Owaku-dani in Hakone, and they say you can lengthen 7 years of your life-span. So many people want to buy there.
Only the shell is black, the filling is as same as nomal eggs.
Sulfur and iron included in the hot water bond to the shell, the color turns to black.