Enemies don't bleed red for the most part, which leads me to believe it's blood. But I'm not sure how I get some of the splatter on my character... There are times where it's a strip or patch, which makes it seem like it was received from a hit.
Damage negated depends on the shield. Every one has a different negation % for the different damage types, with anything less than 100% bleeding through to health.
what happened to the good ol' days when you mostly lost stamina when you blocked