Why does the use of Haki turn part of your body black? Before the time skip, characters like Garp and the Priest of Sky Island, used Haki to deal damage to Luffy via a blunt attack, which would have been impossible otherwise. Also, Haki was used by both sides during the Summit War. None of these people turned black. 

But after the time skip, when Luffy used Haki against Hordy, he partially turned black. And during his fight with Z, they both turned partially black. Is this a special type of Haki? Is there any reason why they change colors and if so why black?
Luffy's Haki ability is called Busoshoku: Koka. Koka means hardening. Luffy's body is made of rubber. Oda is quite clearly referring to hardened rubber, or vulcanized rubber, which is black. (Compare images below)