The Dark type debuted alongside Steel in Pokémon Gold and Silver.
Both types were introduced in Generation II, first released in Japan in 1999. Their addition changed the battle system by giving players new offensive and defensive relationships. Dark-type Pokémon and moves were especially important because Dark attacks were super effective against Psychic Pokémon and Dark-type Pokémon were immune to Psychic moves.
The new type helped address the dominance of Psychic Pokémon in the original games. Steel, the other Generation II type, was designed around high defensive strength and numerous resistances. Together, the two types expanded the strategic possibilities beyond the original 15-type system.
Fairy is a common mix-up because it is also associated with balancing Dragon-type power, but Fairy was introduced much later in Pokémon X and Y. Ghost was present from the first generation, as were Dragon and Psychic. Dark therefore refers specifically to one of the two types added in Gold and Silver.