Dark Pokémon are one of the rare variant of Pokémon in Pokémon Vortex. Metallic Pokémon have 25% more attack than Normal Pokemon
They were made by staff of Pokemon Vortex. Hence they're not to be used on any sites other than pokemon-vortex.com and pokemonvortex.com.
Example:
This Dark Munchlax is a different colour. However, you cannot catch a Dark evolved Pokémon, as all evolved Pokémon can only be caught in the Normal Variant. Dark Legendary Pokémon, however, can be caught.