This week revealed that Mihawk coated his Yoru with haki in order to make it sturdier, preventing it from breaking or even dents or scars (shame, shame) and on top of that, black is the new cool and badass trend nowadays.
Now lets rewind back to Marineford, Luffy vs Mihawk. A certain fellow with a bright red nose gets caught up in a whirlwind of sorts. In order to distract the greatest swordsman of the world to get an escape exit, Luffy needed a diversion. Looking up, he saw some bright red thing, and thus Gomu Gomu no... (grab) Jetto Migawari. Mihawk cut the migawari into two but it "miraculously" reattached itself back. After a brief spat between the rubberman and Rudolph... ahem Buggy, Luffy casually threw him at Mihawk, earning lots of cut from the swordsman who have been noticably annoyed for being toyed around. After all the kingsmen put the many pieces of Humpty Dumpty together again... oh sorry, wrong story. After the many pieces of Buggy were rejoined again, Luffy got his exit opportunity as Mihawk was now stuck having to deal with the devastating tantrums of future-fellow-shichibukai, the great Buggy-sama. Hmm... hold the phone...
Buso haki is what enabling one to bypass a DF ability.
Buso haki is also the one being used to coat / imbued something to add extra power or durability into it.
Mihawk coats his Yoru with haki in order to prevent the swordsman's "shame".
So... (subject title)