There are 2 ways to find out if it is Oda's way of showing us Armanment Haki, or if it is a special kind of Haki that only Luffy and Welgo have been seen using, or even if it is a Luffy only thing, and Welgo has indeed a DV that makes his limbs black.
Have we seen any character commenting on Luffy's/Welgo's blackening since it first happened? Something in the lines of '' why is his arm black'' or something? And also have we seen any other character using CoA haki after we first saw Luffy's limbs turning black? Because if we have seen, and they weren't coloured black, then this means that it is a Luffy thing only, or just a special kind of Haki. CoA may have differnt uses after all, the common and most easy, that isn't drawn black, and another one, more difficult to achieve that produces greater results that only Luffy and Welgo know from the characters we have seen so far (and Rayleigh probably, who taught Luffy about haki in the first place).