Dark does ×1 to Poison

Generations VI to IX, single types. The other way round, Poison against Dark, is ×1.

69 Dark and 83 Poison species among the 1025 this site covers.

Attacker, Dragon and Dark Roaring Moon sprite

damage×1Generations VI to IX

Defender, Poison and Water Toxapex sprite

Toxapex #0748

Dark did not exist in Generation I, so Dark against Poison has no value there.

Strongest Dark attackers

Roaring Moon leads Dark at Atk 139

Sturdiest Poison defenders

Dual types that change the answer