Darkness doesn't necessarily superior to other Logia type DF powers, it's just perfectly suitable for Blackbeard, because he is physically can be superior to many high tiers or even to top tiers, so he can use the darkness to its full potential, while others couldn't. It's like giving a high level sword to a perfect swordsman, but if you give it to a some random person, that person can't use that high level sword. Blackbeard can use the darkness perfectly, but other people probably can't.
I doubt Oda would say Darkness logia > Lightning, Light, Magma, Gas, or Chilly logias. Blackbeard said it's ''the most evil'' or ''most dangerous'' ability, that doesn't necessarily mean ''most powerful''.
https://mangahelpers.com/t/skywalker6705/releases/6199
These are
hierarchically superior Logia abilities;
These two logias are confirmed as superior to even other logia abilities, and ironically these two users fought for the fleet admiralship position, and perhaps these two were also superior to Kizaru's light logia? Who knows, the point is, Darkness is definitely unique amongst the logia devil fruit abilities, and ''the most evil'' too, but there is no evidence to suggest to say that Darkness > magma or Darkness > chilly, they are probably same level if we consider Darkness as hierarchically superior logia ability too. Lightning logia is also considered as ''invincible'' DF power, so that should be hierrarchically superior logia too.
Darkness is superior to Fujitora's gravity, due to having ''infinite gravitation'' ability, which is the reason why it's most dangerous, it can basically pull anything, even the light, or even the devil fruit energy.
As for Gura DF ability, it's also said Gura is the most dangerous, because it's earthquake, and Japanese people know better than anyone how ''dangerous'' the earthquakes can be. Again, Oda never said Gura is the strongest paramecia, he said ''not weaker than logia'', it's logia class paramecia, but there are also other logia class paramecia abilities,
''One of the most powerful'' is important here, it means there could be other powerful paramecia abilities that can rival Gura.
Ope Ope no mi is one of them obviously since it's called ''Ultimate Devil Fruit'' even over the Logia abilities.
So the theme of Blackbeard's DF abilities are not ''strongest'', but it's ''most evil'', Kaido's Devil Fruit ability is a complete package, defense (impenetrable skin) + offense (fire breath, tornados etc) + speed (due to flight), in my opinion it doesn't suit Blackbeard's ''most evil'' theme, if Blackbeard is going to get a zoan, it should be dangerous or most evil like darkness and earthquake.
''Dragon' is not exactly like that, it's definitely one of the most powerful, but Oda can make a more dangerous mythological zoan for Blackbeard, and he already said Blackbeard's animal theme is Hippo before so perhaps it could be an animal that resembles hippo and also aquatic.
Even Orochi's Mythical Zoan fruit seems more suitable if we learn its full capabilities, if that fruit gives the ability to survive decaptitation to the user, then that might be the most evil and twisted zoan ability, and its suitable for Blackbeard. There is also a theory that someone made before, that since Blackbeard's jolly roger has 8 bones on it (just like Kaido's jolly roger has 8 bones) and Orochi's Zoan has 8 heads.