From the moment Rayleigh explained to Luffy (and to us) what Haki really is, I believe that every single time Busoshoku Haki is used, Oda draws visual cues for the reader to know it has been used, but it doesn't actually change color in the OP universe. Mainly to prevent too much speculation from our point of view.
The Busoshoku Haki wiki page in a way confirms this by explaining Busoshoku Haki as an invisible armor.
Busoshoku Haki allows the user to create a force similar to an invisible armor around themselves.
Therefore I believe that when Busoshoku Haki is used, the object does not really change colors in the OP world. Not much real proof could be found to the visual changes of Busoshoku Haki except for maybe the fight between Zoro and Monet. When Zoro slashed Monet, both Monet and Tashigi thought the slash was coated in Haki, which it wasn't in the end. So to me that means that the characters can't distinguish Busoshoku Haki usage with visual cues as we can. They can only distinguish it from the after-effect.
Actually after thinking about it some more, the fact that Oda didn't draw Busoshoku as black before the time-skip would proof that it doesn't actually turn black in the One Piece universe. If it would, wouldn't he have drawn it from the beginning? One could argue that the Koju tribe's Haki wasn't as powerful enough to be turned black, but I doubt someone would disagree on this with Dark King Rayleigh. The moment he blocked Kizaru he must have needed some powerful Haki to block that kick and yet it still didn't turn black. Other examples can be seen during the Marineford arc with Marco, Whitebeard, Shanks, etc.
Contra to what I have said earlier and I'm not sure whether this would be considered canon, but I'll mention it anyway. In Volume 1000, Zephyr is nicknamed "Black Arm" for his superior B. Haki usage. Nicknames are usually something you obtain within the OP world as an OP character, so if you would consider Volume 1000, which is a free volume distributed to One Piece Film Z's attendees, canon, then the answer would be that the black color is indeed visible to people in the OP universe.
PS: I don't speak Japanese and I think this is the picture the wiki referred to, but could someone confirm what is written there?