Although Season 1 of Netflix's Daredevil focused primarily on the battle of wills (and, at one point, literal battle) between Matt Murdock and Wilson Fisk, the Devil of Hell's Kitchen himself did take a short vacation from saving his city to help his mentor track something called a Black Sky. What exactly is Black Sky and how does the reveal of Elektra's true nature fit into that? The second season of Daredevil has elaborated on who and what and how Black Sky fits into this world, so be warned, spoilers for Daredevil Season 2 ahead!
First off, let's recap Season 1 just a little bit. In one episode, Matt's mentor, an older blind man named Stick, returns to Hell's Kitchen tracking a weapon called Black Sky, which Stick referred to as "the bringer of shadows." The creepiness factor only escalated when it was revealed that the Black Sky was a boy being transported by an evil organization known as the Hand — which we now know to be the villainous counterpart to Stick's organization, The Chaste. At the end of the episode, Stick killed the boy/Black Sky. So, everything's fine and the Black Sky is totally gone forever, right? Nope!
In Season 2, the war that Stick foretold to Matt arrives in Hell's Kitchen as the Hand is back — led by a resurrected and scarred Nobu — looking for their Black Sky, who just so happens to be Elektra. But, the war isn't simply a physical thing between the Hand and The Chaste, there's also a metaphorical war within Elektra. See, she has spent most of her life training under Stick's tutelage to kill the Black Sky, while fighting an innate violence that led her to kill one of her fellow Chaste trainees in a fit of rage. So Elektra must choose between what she's learned from both Stick and Matt and her supposed destiny as the Black Sky and the leader of the Hand.
Although Black Sky isn't a term from the Marvel Comics, Elektra has a long history with the Hand in the comic book mythos. On Daredevil, Elektra chooses to side with Matt, but dies in the battle against Nobu and the Hand. However, we see members of the Hand put Elektra in a stone coffin (which the organization had previously been filling up with the blood of children — as if you needed anymore reminder that these guys are evil) presumably to perform some sort of ritual and bring her back to life to become their Black Sky.
So you may be asking how this all ties into the question of what exactly the Black Sky is, and the answer is that I'm still not sure. It could be a weapon of some sort that imbues a specific kind of person, like Elektra, with special powers. Or, the Black Sky may simply be a person foretold to lead the Hand to victory. Another option is that it has something to do with the Hand's mystical history (even if Matt Murdock doesn't believe in that stuff). For more answers about the Hand's Black Sky, we'll have to wait and tune in to another season of Daredevil!