User blog comment:The Bonehead Skipper/Color of Armament/@comment-3079795-20101125183749

Hmmm, seems to me that when Oda was drawing this stuff he realized that even though we can all assume Luffy is using haki we can't really "see" it. . . So he probably decided that it'll randomly make his hand black so that it won't be like "wow he used haki. . . again", it'll be "AWESOME HE USED HAKI AND HIS HAND TURNED BLACK!!!!!!!"