Back in Daredevil, we learn that The Hand is
farming blood from people, and it appears it has something to do with their resurrection spell/ritual.
But then in The Defenders we find out that The Hand's
secret ingredient for immortality is dragon bone dust. But if that's what they needed, what was the point of all the blood?
They were used for two different rituals.
The "substance" being mined in The Defenders is what The Hand, and presumably the masters of K'un Lun, have been using for centuries to keep themselves alive, and what allows their resurrection to happen. What's under New York City appears to be the last major reserve of the substance outside of K'un Lun, so the Hand is eager to get at it, because their supply is running low. Prior to that, they must have had stockpiles of it from previous mining operations, so they were content to leave what's under NYC alone until they needed it.
In Daredevil, we are seeing the efforts of what seems to be Alexandria's faction of The Hand, focusing on her vision to bring about the Black Sky. (Note that Madame Gao doesn't ever seem to take a direct role in the events of Daredevil) This is a special ritual, different from the resurrection ritual, and requires more resources. Not only does it require the blood being drained from the captives, but it also uses up the last of the Hand's reserves of the substance. Alexandria believes the Black Sky will help them finally return to K'un Lun, so she's willing to put all of those resources into a single ritual to bring back the Black Sky.
