No, I don't mean the Shakespeare play. I mean the board game in which pieces change from black to white and back. This happens in card games
far too often.
Nearly every monster-based card game has some set in which a few cards from previous sets 'go dark.' For Pokemon, the first set of this sort was the Team Rocket expansion (which came after Fossil); for Yu-Gi-Oh!, Phantom Darkness (although one could argue about Pandora/Arkana's variant Dark Magician). Digimon had a slew of type and color variations from the get-go - MetalGreymon was originally a
Virus type.
So, why do companies do this?
There are a number of reasons. The most prominent one is that everyone, at some point and on some level, wants to see heroes go dark. That was why Toei/Bandai changed MetalGreymon's typing to begin with; in the anime, Virus-Type had been
almost equated with evil. MetalGreymon's lost identity as a Virus-Type later made a cameo in Season 2. When there are stark 'good' and 'evil' sides, as there are in many of the series that Japanese card games are based off of, there's always a question of what would happen if the protagonist turned to the dark side a la Star Wars.
Unfortunately, this can be cumbersome in card games. For example, in the Team Rocket set mentioned above, one could only evolve Dark Charmeleon into Dark Charizard, Dark Wartortle into Dark Blastoise, and so on. A similar thing happened with Gym Leader and Team Aqua/Magma Pokemon.
Oh, how Gen III abused its ability to make things with more than one type.
Many Team A/M Pokemon were, you guessed it, cross-typed. They were often Dark/Whatever they usually were. If nothing else, they no longer had 'dark' in their names, and had more evolutionary options. I realize that Dark-Type wasn't around when the TR set was released; Nintendo updated that idea in "Team Rocket Returns" using actual Dark-typing, as well as slipping 'Dark' in the name. There was also an outlying Dark Celebi in EX: Hidden Legends.
OK, whatever. Using Dark like that is understandable, right?
Enter Delta Species. The idea behind it was that some scientists in the Holon region (which is not accessible in-game) were trying to locate Mew via electromagnetic waves. This caused mutations in the Pokemon of the surrounding area - namely, type changes.
Game-wise, this was done to make sure that relatively underpopulated types - Fire, Electric, and Steel - got more coverage. Delta Pokemon retained all other attributes of their former types - that is, a fire-type Pikachu (that did not exist, but as a hypothetical example) would still be weak to Fighting as opposed to being weak to Water. It would make a mono-Fire deck a bit harder to counter. Smart idea.
While the Pokemon Company meant well, they really could have done more with the whole 'Delta Species' concept. For example, there was a Grass-Type Dragonair; wouldn't it have been sweet to see a green Dragonair using Razor Leaf? Wait a sec- Dragonair don't come in green.
That's the thing that made Delta Species suck: They diversified the attacks well enough, but did not do so with the art. Black, Dark-type Shiny Charizard was a good move; they also made a Fire-Type Gyarados to coincide with its alternate red coloration. They were already breaking the rules with Delta Species. Why did they not go one step farther and change
colors as well? It would have been interesting, challenging, and engaging to see, say, a Milotic using a Fire attack.
While this may seem like an unwise move on Pokemon's part, Yu-Gi-Oh! is the master of atrocities when it comes to giving creatures alternate forms. Othello is no exception to this rule.
Dark is one of the strongest types in the game. It has been since the days of the very first set, "Legend of Blue-Eyes White Dragon." Despite the set's name, back in those days, everyone was running Fiend/Spellcaster decks (called F/S Beatdown) with Blue-Eyes splashed in if present at all. It only got stronger when Chaos came around (removing 1 Light and 1 Dark Monster for a 3000/2500 BEAST -> Mokey Mokey + Skull Servant = GG). If anything, all other types were unbalanced in comparison.
Then "Phantom Darkness" came out. Then, everything else REALLY became weaker than Dark-Types.
In Phantom Darkness, the super-strong "Dark Boss" monsters were released, along with powerful support cards like "Allure of Darkness." These cards were not just good- they were AMAZING examples of type support done right. For example, Dark Armed Dragon required
exactly 3 Dark Monsters in the Graveyard to be Special Summoned. Once on the field, it could wreak havoc with its amazing 2800 ATK and monster-destroying power. It stuck to its grits as a 'retrained' Dark Armed Dragon Lv7, bearing similar stats and abilities. The other Dark Bosses generally followed suit (with the exception of Dark Simurgh, who outshines its non-Dark counterpart in every way EXCEPT stats).
If Dark-Types had really needed the help, this would have been welcomed. Atlantis ("A Legendary Ocean"...up yours, 4Kids) and Daedalus were much-needed Water-Type support. Dark already had a plethora of things making it good. It did not need the help that nearly every other element did.
So, why did Konami do that? For the same reason they do everything: "Because it's cool." Never use
Invader Zim logic in the real world. You'll make money, but it will not have a pretty effect on the metagame.