Almost since the beginning of season 2, I thought "Black Sky" was a Hand protocol akin to the machine war in The Matrix. To literally turn the sky black. This would be an introduction for Blade into the Netflix Universe. And when you think about it, it isn't that much of a reach.
A black sky means no sun. Blade is a vampire. Vampires are weak to sun.
The Hand was collecting blood/plasma from the kids DD saved. Vampires needs these fluids to survive.
"Black Sky" was in a coffin shaped like an urn. Coffin - vampire - Blade.
Blade made all 3 major debuts in the various formats in other characters canon; in the comics Blade is first introduced via Dracula's book. In the cartoons in Spider-man, and in video games in Ultimate Alliance. Even his first action figure was debuted in a Spider-man line called "Vampire Waves".
Only makes sense that another property would serve as his Netflix entry, and with Luke Cage and Iron Fist getting their own series, he'd be a perfect anti-hero injection. He could think LC was a vampire like him, or LC could think Blade is a new wave meta. Blade and IF could be sparring buddies or IF could want to soul purge Blade's vampirism using ancient Kung Fu mysticism. The Hand could be controlling Blade as a mindless drone to kill opposing forces. All sorts of tie in material.