So, what is the Dark-type?
Most RPGs have some sort of an Evil element, either for villains, or for users. While the moral alignment of the RPG generally shifts the balance towards either holy magic or dark magic, most RPGs have both for common player characters regardless of the moral alignment of the game.This helps some "good vs. evil" stories to have some level of unintentional gray in them, as the moral alignment of the magic you use generally translate to your character's moral alignment.
Pokémon did not have this initially, not because there wasn't much moral ambiguity in the games(which of course, there wasn't), but also because of the games' design. World of Pokémon is virtually the same as real world, and the monsters in it are, well, were, not god-beasts, but regular animals with magical animals, or personification of elements and emotions. This makes an evil type or a light type rather difficult to create. Nonetheless, Dark-type was needed for a few reasons
The Prehistory of Dark-type: The Evil Type:
As everybody knows, Dark-type is known as the Evil type, or "Aku" type. The general speculation is that Dark-type is not "evil" in the way one would understand in a conventional RPG, but rather "evil" in an Asian sense, unbalanced, underhanded, greedy etc. This idea seems not to be accurate, as the original Dark-types included a witch crow and a demon dog. The more likely origin for Dark-type's themes is not Asian philosophy, but rather the 90's aesthetic. In the dark age of comics, many demons and hellriders were prominent in popular culture, and comic heroes would involve themselves in morally ambiguous storylines. This dark and edgy aesthetic was also what made Pokémon successful compared to its competitors in the Western and global market, but Pokémon series did not have a type to capitalize on this, and Ghost and Dragon were designed to be rare in the first generation.
Another major element was world building. As described above, Pokémon missed the moral duality presented by game mechanics that is prevalent in other RPGs. It existed in the story, because of the game's nature as a kid's game that pays focus on adventure, but it did not exist in the gameplay, so you didn't have anything that would clue on you about evilness of a given enemy outside of visual and audio presentation. This meant that evil teams would have to use a close approximation to showcase their villainy by using Poison and Normal-types, and mischievous Psychic-types. However, with Dark-type's introduction, we now have a dedicated type for villain teams, which also retroactively allowed the evil teams to focus on another type, with Dark-type serving as a support type. Team Rocket no longer had to use Poison-types because they were the evil type, they used them because that was now their type orientation over future villain teams like Aqua, Magma, Flare, Plasma, Skull etc.
Second major element about world building was the day and night cycle. Despite being introduced in games like Dragon Quest 3, original Pokémon and the third generation did not have day and night cycle to immerse the player. However, Gold and Silver introduced this to change up the wild encounters and some trainers, as well as the overworld view. This was due to the GB Color, which the developers wanted to showcase. While other types that could have been retrofitted to nocturnal species existed, they were not common enough.
Third was a color issue. Original games did not have a type that would fit for the Black color, despite being a primary color. So, Dark-type fixed that as well.
The last issue was the rebalancing of the type chart. Psychic nerf is the obvious reason, so is Ghost's rarity, but Dark-type's main purpose shines in coverage. All other major coverage types have either a type that is immune to them, lack powerful moves, or resisted by a lot species. Dark-type is not resisted by a lot of common species, and is not rendered null by a type or in the future by an Ability. This idea of an universal coverage type was also one of the reasons Dark-type was created.
The History of the Dark-type:
Original Dark-types were an elaborate joke. Most Dark-types introduced in this generation were physically oriented, except Houndoom, in spite of the fact that Dark-type was a Special type. Not only that, the only Dark-type you could obtain before Elite Four was Sneasel. Everything else was exclusive to Kanto, and most of them were available only in Night. Not only that, Dark-type moves were largely weak, and the only relevant one, Crunch was only properly distributed in Crystal through Egg Moves.
The sole TM for the type was Thief, a weak move that allowed you to steal opponent's item. Other moves included Pursuit, which doubled only when player switched out, which didn't happen in NPC battles, Feint Attack which was pathetic, and Bite, which was originally a Normal move. While Dark-type was granted an Elite Four member, Karen had to buff her team with Gengar and Vileplume to fill in the amount of Dark-types.
Third generation was a copy of the second generation, but about %30 cooler. Vast majority of Dark-types this time around were early game and mid-game fodder, some more rarer than others. This generation did not provide a Dark-type pseudo-legendary, but it provided a successor to Arcanine, Absol. This generation also provided a Dark-type with no weaknesses, Sableye, and a Dark-type secondary rodent in the form of Mightyena, which would influence later Dark-types.
In this generation Dark-type got a more support orientation. The sole offensive new move was Knock Off, which was the second weakest Dark move in this generation. The rest were TM support moves like Taunt, Snatch and Torment, and common Dark supports like Memento and Fake Tears.
Fourth generation introduced the physical-special split, retroactively making Dark-type actually playable for the first time. The new Dark-types were still early route fodder primarily, but they had higher BST than than their third generation counterparts, despite being predominantly second stage. This generation also updated two weakest Dark-types, Murkrow and Sneasel by giving them powerful evolutions, and introduced the first Dark-type legendary, the Mythical Darkrai.
With the physical-special split, Dark-type was changed into a Physically dominated type. However, Dark-type also got the powerful Dark Pulse, a common Special move. Other moves were Nasty Plot, a Swords Dance clone for Special Attack, Fling, the ultimate Battle Facility enemy move, and Switcheroo, a Dark-type Trick clone, among others.
Fifth generation was a short-lived renaissance for Dark-types. Dark-type got another pseudo-legendary, another marketable carnivore, another pseudo-rodent, and tons of mid-game and late-game species. Sadly, the overpresence of Fighting-types overshadowed the Dark-types, but Dark-types finally got associated by abilities such as Prankster, Moxie and Intimidate.
Sadly, the type didn't get much in new moves. Snarl was originally event-exclusive, Quash was rare and pointless, and Foul Play was largely for defensive species.
Sixth generation was where the type died for good. Fairies were immediately retconned as the new nemesis of Dark-type, replacing the old battles between Psychic and Dark and Fighting and Dark, and this new type was quad effective against both Scrafty and Hydreigon, the two of the pivotal species in the previous generation. Dark-type was hampered heavily in the common species department, as both Pangoro and Malamar were terrible, but the legendaries were decent, and Megas were cool. Greninja was, of course, one of the most broken species in the metagame.
The new moves were signature only, but Knock Off got a great buff that made it a primary coverage move.
Seventh generation introduced only two new Dark-type lines. The rest were Alolan forms, so Kanto had proper Dark-type distribution. The new Dark-types were terrible, though Alolan forms were cool.
The moves introduced in this generation were relatively rare, and Sucker Punch got a pointless nerf.
The Overview of Dark-types:
Dark-types were the ultimate expensive garbage in the previous generations, but after the introduction of Fairy-types, Dark-type became just a rare early-species type, as a bad offensive copy of Poison. Dark-types in the past had some potential as a quirky defensive type with neat resistances and immunities, and as a decent STAB type with potential secondary types, but with the introduction of Fairies, Dark-type was rendered obsolete.
Dark-type's main gimmick before Fairies was its lack of weaknesses. While Bug and Fighting were common types, they could have been removed by addition of other types, like Ghost, Flying and Poison, but with the introduction of Fairies, this is no longer the case. Similarly, Dark/Dragon and Dark/Fighting, two decent types with great resistances, are now rendered worthless thanks to their quad weakness.
As Dark-type is a relatively rare type despite being common in terms of encounter rates, and is a physically oriented type despite being historically a Special-type, Dark has not completed its full potential yet, but since it lost main gimmick, there is really no reason to bother with it. It is just a generic bad guy type now, and thus, it is designed to suck, regardless of the pseudos it might have.