What if white blood cells become dark brown or black?

The colour of blood is because of the red blood cells, not the WBC. In various hemat tests colour of the nucleus is visible of WBC which is purple to pink in colour. read more

The pus we see in infected wounds, abscesses, and the like would be dark brown or black in colour. Otherwise, assuming that the function of the white blood cells does not change, everything else would be more or less the same. read more

Dark-colored blood, called venous blood, is dark because it has become deoxygenated after having traveled through the body before returning to the heart drained of oxygen. Blood leaving the heart, called arterial blood, is pumped from the lungs back to the heart by the pulmonary artery, and it contains a large amount of oxygen for delivery around the body. read more

Blood that you normally see after a cut is venous blood which has the lowest oxygen levels in the blood vessels. Occasionally in certain light, or if the oxygen level is very low, the blood can appear almost black. Blood infections are very serious but cause more noticeable symptoms such as fever and loss of consciousness. There are certain blood poisons that can cause blood to become a rust color, but usually not black. read more