User blog comment:Pacifista15/Explosive Punches/@comment-3493137-20111028112118/@comment-1595565-20111028160214

Luffy's arm going black seems to be a reference to vulcanization, the process of hardening rubber. Rubber in its raw form is sticky and not easily applicable in most cases. Vulcanization solidifies it further and makes it smoother, turning it black. Luffy seems to be doing that, or something to that effect when he uses busoshoku koka.