It's not just about possessing a black blade. The greatest benefit will come from creating (forging) one.
First, there are known benefits to having a black blade regardless how it is obtained.
- greater defensive power. Black blades are unbreakable so possessing one means the user will never have to worry about their sword breaking so their defensive power will increase.
- greater offensive power. Black blades increases AP because of the stronger blade. When Zoro first used shisui his attacks from shisui were stronger than his other swords attacks.
- better haki preservation. Possessing a black blade is like always coating your sword with a strong blackened CoA haki without needing to actually do that. When haki is needed to create stronger offensive or defensive techniques the user won't need to use as much. Of course, black blades don't actually contain haki so haki will still be needed to counter DF power.
Possible benefits to possessing a black blade.
- I believe black blades are heavy. Creating a black blade increases the weapons weight. Zoro stated Shisui was heavy compared to his other two swords. A heavy blade means the attack will have more weight. That increase in weight will increase the attack power. The user will grow physically stronger from needing to train to control the heavier sword.
- I believe black blades don't possess cursed/dangerous power. Creating a black blade will remove the curse/dangerous power the sword posseses. I don't have anything to support that but I do find it interesting the only difficulty Zoro had using shisui had to do with possessing a black blade, nothing about shisui.
Let's look at the possible benefits one can obtain from creating (forging) a black blade.
At this time nobody knows how black blades are created but the manga has given us some insight so we have some information to work with.
- largest/strongest CoA haki amount and the best CoA haki control in the manga or near it. There is enough information to confirm blackened haki is needed to create a black blade. But just possessing that isn't enough to create one. Since Zoro has mastered enma he has started to create a green aura around his swords when he's using KoH techniques. We have never seen that before from any character. I believe that is Zoro adding an incredible large amount of controlled blackened haki to his swords and that is the first step to creating a black blade.
- incredible powerful CoC haki and control. We know just to fully master a sword the user must bend the sword to their will. If you create a black blade you aren't just bending it to your will you are changing the sword. That can only be possible if you can completely dominate the sword.
- I'm sure I'm missing one or two more things needed to create black blade.
Of course, a black blade creator will receive a huge increase in CoC strength. Having the skills/power to create a black blade and creating one will make the user stronger. As they grow stronger so will their CoC strength. Similar to what happened to Luffy when he obtained awakening.