While there is still little known about the Black Sky, there have been several comparisons made between how it is revered by The Hand and the creature known as The Beast from the comics.
Having the Black Sky be connected to The Beast (or perhaps be an MCU version of it) would not only give Elektra a reason to be “bloody” during The Defenders, but it could also provide an opportunity for comics easter eggs as well.
The Beast was the primary antagonist in that story, taking control of Daredevil and placing him in control of The Hand.
Marvel’s The Defenders premieres on August 18, 2017 on Netflix.