>associated with blood

sanguine is one of those words which has developed somewhat opposite senses:
1. bloodred
2. bloody; bloodthirsty, murdurous (sanguinary); ruddy
3. having blood as the predominant body humor
4. confident, optimistic

(not exactly opposites, but murdurous and optimistic seem on the further reaches of some sort of scale)

anyway, I suspect that all relates to the popularity of red as a color.