Back on topic
I have a few Kita stones. Each is different than the others in its own way. I don't get 'black' slurry from any of them when honing a western razor but maybe I would if I sharpened a chisel or Kamisori or something. I've read where people have claimed that Kitas are 'generally' softer than Asagi, but then I have seen many Asagi stones that were being sold as LV 2 or LV 3.
So - I don't know what to think other than each stone is it's own thing. Generalizing about Jnats based on color might not be all that helpful.
Still - the hardest stone I have is an Asagi - but I'm not prepared to believe there aren't any Kitas that would be just as hard. I just passed on an offer to buy an Ozuku Kita that was claimed to be as hard as anything out there. I'll probably never know unless I change my mind and buy that one too. I chose the Mizu Asagi over it based on size mostly.
The Nakayama Kita used to be/still is (judging from what I've read, I have no hands-on or first-hand knowledge here) a highly coveted stone. My guess would be that these Kitas were known to have a good mix of the best qualities. IMO - the really really hard stones are very very hard to work with. For me - a bit of softness goes a long way.
Theoretically - one would be able to produce the finest edge on the hardest stone. I think this is where the skill-level variable speaks volumes; without the skill, the resulting edge will be flawed.