No one knows what the black sky actually does as it has never been explained and there’s a reason for that “bait and hook” even. “Stick” said that The Hand has never been able to get their hands on one until Electra.
Remember that Electra only recently found out that she was one of the black skies. Nabo said that they are extremely rare so there is more than one. That kid being killed at the shipyard was one of them. The only thing we know is that they are not immortal; they can be killed.
I’m not sure if we will ever find out exactly the purpose of the black sky. It has to be more than we currently know of because Electra has been beaten in battle and she has also died, so there is something untapped inside of her that needs to be released for the full potential of the weapon.