User blog comment:AYET/Secondary ability of devil fruit powers/@comment-4830969-20120211102959/@comment-94.70.71.100-20120214155345

luffy didnt vulcanize his body. that black colour was haki.when he fought the kracken he used haki plus gear third. besides in vulcanization you need to add extra materials in order to harden the rubber. it was haki.