Quite obviois if you read all translations.
Franky is surprised Luffy is using Haki meaning he had no idea Luffy was capable.
If Franky had seen Luffy's fist turn Black, he'd know Luffy was a Haki user and not be surprised.
Sorry, but you're lacking critical information.
If you read Thriller Bark arc or this Panel, we're given the definition of a Black blade.
A Black blade is a blade that's very hard.
If you understand what Mihawk is saying here, he says:
If Zoro learnt to infuse his Sword with Haki, his blade wouldn't break or get nicked.
Haki makes things hard. If a Sword gets imbued with Haki, it becomes hard. That essentially makes it similar to a black blade, not literally a black blade.
Furthermore, if you remember Zoro's reaction during the Wano arc about Blackblade...
He's wondering about how Black blades are formed.
If Mihawk was explicitly saying a Blade becomes black with Haki, Zoro wouldn't be wondering about anything, he'd know. He clearly doesn't know how it's forged meaning Haki is not what makes a blade becoming Kokuto or Black literally.