Spoiler:
This season, the main "villains" are creatures called Leviathans. They're the ones I believe you saw in the beginning recap of the ep, with the huge mouths filled with sharp teeth. Sam and Dean have been working on finding a way to kill them, with limited success. But the connection with what Chronos said, is that Leviathans do not have blood, they are filled with black oozy stuff instead.
On a recent episode, two hunters meeting, took the precaustion of cutting themselves to prove to the other they were human. Because until they open those huge mouths, Leviathans look like ordinary people.
On a recent episode, two hunters meeting, took the precaustion of cutting themselves to prove to the other they were human. Because until they open those huge mouths, Leviathans look like ordinary people.