User blog comment:Firefist553/How will they beat him?/@comment-77.49.160.106-20111207091358/@comment-4477846-20111207094954

the reason why luffy turns black isnt because his haki is stronger, but his rubber body vulcanize resulting in the rubber hardening and turning black.

the dying old man was the strongest man in the world so i think his haki was under control.

http://en.wikipedia.org/wiki/Vulcanization