How do you say dark fire in Japanese?

暗い火事 (kurai kaji) would mean "Dark Fire" in Japanese.

Improvement:

火事 (kaji) means fire, but in a different sense than the element 'fire' itself.. it's used when E.g you say 'The fire burnt down the building'.. in a meaning of 'conflagration'.

暗い火 (kurai hi) is more suited for 'drak fire', since 'fire' means 'flame, blaze' here.