User blog comment:Dettles/Questions about haki/@comment-8823907-20130620114043/@comment-8823907-20130624175552

Well oldmanzangetsu the most likely explanations so far is that Oda didnt think of it at the time. All black limb haki forms have been seen after the timeskip. Rayleigh could fight kizaru without using the black limbs and marco aswell, jozu and WB could too and Vista also + Garp.

However it could be a sign of intensity as well. I still believe that before the timeskip Oda didnt think of it but after the time skip there have been moments where armanents haki has been show being in use but didnt turn the limbs black (such as peckoms striking caribou, Luffy grabbing Cesar or Smoker). So my conclusion. The black limbs didnt exist before the timeskip and after the timeskip are directly related to the intensity of strength the users haki is capable of.