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What I mean to say is that the Black Death of the 14th century still devastated Europe, but the culprit in this alternate scenario is not Yersinia pestis, the bacterium believed to be responsible for the plague.

In OTL, the Black Death was Black because one of the plague's most conspicuous symptoms is the bubo, a swelling of the lymph glands. Each bubo was as big as an orange and, you guessed it, black in color.

I'm picturing an alternate scenario where the medieval plague was called the Black Death not because of the buboes, but because the victim's blood turns black, resulting in some ghastly discoloration before death.

The simple question is, how do I change a blood cell's color from red to black?

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  • $\begingroup$ Possible duplicate of Other blood colors $\endgroup$ – Separatrix Jul 5 '16 at 13:41
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    $\begingroup$ I do not feel that this is opinion based, especially since the question is essentially "What process of disease would turn blood black instead of red?" If there is a way for this to happen in a science based manner, it would be fact, not opinion. You may think that it's a poor question, but that's not a close reason. That's an opportunity to leave a comment on why you think it's poor and how to improve it. $\endgroup$ – AndyD273 Jul 7 '16 at 16:22
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The red of red blood cells is caused by Hemoglobin, an iron bearing molecule which is redish when oxidized and more bluish when not oxidized. If your fictitious pathogen were to attack hemoglobin chemically, the red color would vanish. One potential solution would be to create Iron(II,III) Oxide, which is a black chemical often used as a black pigment for painting.

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    $\begingroup$ Note that since hemoglobin's function is to transport Oxygen within the bloodstream turning the blood black in this fashion would result in death within minutes to seconds as all of the body's tissues including the brain and the heart would be oxygen-deprived. Truly black blood would only be found in corpses. $\endgroup$ – ApproachingDarknessFish Jul 5 '16 at 1:54
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Blood is pretty black by itself in the first place, especially outside of the body. Blood packs are almost black and it would be hard to tell the difference under the skin. Outside of the skin it would start to dry up into a black scab anyways. Hypovolemia (plasma loss) would be cause blood to deepen in color significantly and could cause blood to appear dark from the outside and turn black more quickly outside. The reduced oxygen-carrying ability would also cause gradual necrosis in tissues, turning those black as well. If a process where water was removed from the bloodstream while maintaining blood cell count, this would provide a convenient way to turn blood black. Turning heme into Fe3O4 is a bit difficult since breaking down hemoglobin requires multiple steps and would be difficult for a single bacterium to do.

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An alternative to black blood would be for the flesh itself to turn black because of necrosis (gross link warning). There are a number of bacterium that cause necrosis, and one could be modified for your needs. Since bleeding and other bodily fluid related "treatments" would be common, bacteria could spread quickly. Make the bacteria ravage the skin quickly at a superficial level first (causing the skin necrosis to spread throughout the body first) and only then go on to infect the remaining tissue and organs, and you have a body that is black with necrotic tissue while still alive.

Slow-moving necrosis and gangrene were known of during the medieval period (though I am not sure of what extent they were understood), usually associated with open wounds and ulcers and treated by maggots. However, it was likely to be fatal or debilitating, as even as modern as the civil war gangrene was a serious problem.

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Does it have to turn the blood black within the circulatory system or tissues, or just cause blood-related symptoms that could plausibly result in the name 'the black death'? As @Zxu's answer notes, blood goes pretty much black anyway when it starts to break down outside the circulatory system. For instance, severe (and usually fatal) cases of yellow fever are characterised by black vomit. In Spanish yellow fever is known as 'vomito negro' for this reason. So you could simply postulate either a world in which a temperate Aedes mosquito becomes well-established in Europe or one in which yellow fever virus is transmitted efficiently by an alternative vector (fleas, etc) or route allowing it to spread extensively.

Alternatively, you postulate have a viral haemorrhagic fever resulting in extensive subcutaneous haemorrhaging, which would cause a sort of bruising under the skin. Not very black (one virus I work with does this to the tongue of sheep and other ruminants, and is called 'bluetongue') but close enough that it could plausibly become known as 'black death'.

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