Well, uhh unfortunately, you don't get any Dark types in the near routes of Hearthome, but since you use Dark, I assume that your team consists of a Murkrow/Honchcrow and a Stunky/Skuntank. However, in Hearthome city, there is a NPC that gives you an Eevee, which can be evolved into an Umbreon, if you want a defensive Dark type.
Edit: the next Dark types can be found in Route 214 (Houndour) and MT Coronet (Sneasel), while in the Great Marsh, you can get a Skorupi and evolve it into a Drapion