With Blake drawing the flame pillar rune formation, the same one that he drew about an hour ago to combine with the whirlwind rune formation spell and create a fire tornado to kill the vultures, the humanoid demon finally took the full brunt of Blake's attacks without being able to weaken it beforehand.
Blake intentionally used the flame pillar attack, one that had pure fiery red flame inside it rather than the dark flame because of the humanoid demon's ability.
From his tests earlier, Blake found out that even though the humanoid demon was able to weaken his spell that contained a significant amount of darkness elements in them, the humanoid demon couldn't do anything against a spell that didn't have the darkness element as the consequential part of the spell. Evident from the humanoid demon being unable to do anything other than defend against Blake's ordinary earth spike before.