BlackFlame + Dark Effigy

BlackFlame makes my ignite to be a chaos dot

Dark Effigy targets units with chaos dot

Why those two wont work together?

It seems just a missed opportunity for GGG here.

Technically, it doesn't do anything that would change Ignite to become a Chaos DoT. It converts, not changes, the Fire damage into becoming Chaos. The distinction is when it actually does Chaos damage - since it's not initially Chaos, it still counts as a Fire Elemental Ailment, even though its damage being converted means its damage would now scale off Chaos increases instead of Fire.