Of all the blood dyscrasias, Atrabilious Blood, or the amalgamation of blood with morbid black bile, is the most pathological.  That's because black bile is extremely toxic.  Blood that's been corrupted by it will be a deep, dark red, thick and sluggish, stagnate or clot easily, and stick to blood vessel walls.  Atrabilious blood is a major cause of stagnant blood, and is also associated with clots and embolisms