Not Dagon, Marethyu. Dagon just talks about the black aura
And I don't think the rainbow aura is the rarest. A rainbow aura just means that one would have red, orange, yellow, green, blue, indigo, and purple in their aura, and there is every likelihood that many people throughout history have had rainbow auras. In my opinion, the black aura is the rarest, because only Death himself even has a chance to possess it in the entire series.
Marethyu is actually Josh after combining the four Swords of Power, so, "Death" would have to have a golden aura.
Marethyu is dead, the hook sustains his life force, and he does not have an aura. His heart does not beat, and he does not breath. A black aura was possessed by a Great Elder with an unknown name. Rainbow auras are not the rarest, Iris and maybe 5 others have possessed it. There is never a hint to Marethyu having a black aura, it is very stereotypical that someone would assume that, and yes Dagon did only speak of it.
when souls die they are made of aura. whatever aura they started with is what they are made of in the afterlife. no matter what job they have when they die, Deaths aura can really be anything he chooses even a blue green