Dark Pokémon are one of the rare variants of Pokémon in Pokémon Vortex. Dark Pokémon do 25% more damage than Normal Pokémon.
They were made by the staff of Pokémon Vortex. Hence, they are not to be used on any sites other than pokemon-vortex.com and pokemonvortex.com.
Example:
This Dark Munchlax is a different color. 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.