Nightmare Moon and Nightmare Rarity are the product of the
Tantabus and its host, fused together in a very seamless way
Technically, the Tantabus was created by Luna after the Nightmare Moon incident to punish herself – it only came into existence well after Nightmare Moon was defeated. At least going by comics canon, I think you might mean the
nightmare forces?
Pedantry aside, I agree with the overall point. The difference between Luna and Rarity on the one hand and Nightmare Moon and Nightmare Rarity on the other is like that between
Stygian and the
Pony of Shadows, who also get separate blue tags – the latter forms are the result of the original pony fusing with something else and becoming, in a sense, a new being. Sunset Satan and Midnight Sparkle are still the same people they were before, just a lot more powerful, a lot less inhibited and a fair bit less sane.
That all being said, and changing the argument a tad, but
@Dogman15 ‘s comment made me wonder:
shouldn’t principal celestia and
vice-principal luna be blue tags? They’re are separate as Twilight and Sci-Twi… are they separate enough in terms of character and use to warrant making separate blue tags?