Seems like, in order to make permanent black sword, you couldn't just "
frequently" or "
usually" using sword and imbue it with strong haki.
You have to "
always" using that sword in order to make it permanently black.
Have you seen Mihawk using his punch to beat opponent?
Not saying he couldn't, but he never use it from what we know.
Even when he want to give his opponent advantages, he use small sword (or knife) instead of going barehanded.
"Frequently", "Usually", or even "Mainly" isn't enough to make permanent black sword.
It has to be your only way of fight (except that you don't have sword or anything to be hold).
This is why making a permanent black blade is difficult and it is considered impressive since it needs total dedication to use swords as the only way of fight.