Haha, yep, I found the explanation in another walkthrough. It's almost impossible to detect that those blood marks are a clue for the solution of this puzzle, even having the hint from the hint system. I also had to look up the location of the key, which was in a spot that was completely black.

There's a pile of books in that cabin and one of them has some blood smears that's supposed to be a clue.
It's one of those clues that aren't helpful until you already know the answer lol

How do you know this combination?