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?