Be glad. Those elemental chests usually give good loot. You fill a dark chest by killing anything purple ingame - monsters, heroes in raids, even titans. When you hit 150, open the chest and it will be replaced by the normal titan chest.
They can show up replacing any of the the three wanted chests, roughly one per 30 of the ordinary ones, in any color, red, yellow, green, blue, purple, and are each filled with the same color enemy.
There’s a thread listing the best and most efficient places to kill enemies for each chest.
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