Dark Pokémon evolve from Basic Pokémon. Once you evolve a Basic Pokémon into a Dark Stage 1 Pokémon, it CANNOT evolve into a non-Dark Stage 2 Pokémon, only a Dark Stage 2. "Once you walk down the Dark Path, forever will it dominate your destiny" - Yoda the non-Pokémon.
Source: WotC Chat (2000-04-13)
How do I evolve a Pokémon?
You just play the evolved version of that Pokémon on top of the Pokémon you want to evolve. In other words, you have to have the unevolved version already on the table and the evolved version in your hand, and then you just put the evolved version on top of the unevolved version. You can evolve an Active or a Benched Pokémon.
Source: WotC Chat (1999-09-30)