My thinking there is to merge the rat-plague from Dishonored with the tainted-blood plague from Bloodborne. From what I've heard of Bloodborne's lore, the plague twists people into monsters, which Dishonored sort of has with the Weepers, only Bloodborne's transformations are more extreme. The mechanic (in the game) involves keeping your tainted-blood level down to avoid becoming more bestial and gathering pure blood; both work as healing items but pure blood is rarer and more powerful, therefore much more valuable. It sounds kind of like how in Fallout, non-radiated water is way more valuable than dirty water, but both heal you.

Meanwhile, the Darksign curse is a separate thing, merged in with the mark that the Outsider leaves on Corvo and other chosen individuals. I'm not sure if that makes you immune to the plague or not - I'll have to double-check if it made Corvo immune to the Outsider's rat-plague for instance.