[Fern]

How infected does someone have to be before they notice they have black blood, as in a rough estimate of how many dusks would have to be in a person before that type of thing becomes visible?

How long does one live roughly if they have black blood?

Would further infection speed up this black blood till the victim dies?

We know that dusks can infect things like animals etc but can they infect things like plant life and such?

1) A small pack of Dusks is enough to begin defiling the host's blood.
2 & 3) Further infection can speed up the magic contamination of the victim's blood, and it can also make them more likely to die faster. Emphasis on 'can', as while it's what usually happens, it doesn't always do.
4) I don't think they can infect normal plants per-say, but they can infect plant-like monsters (I.E Vampiric Legume, Mad Vines.)