This is a "feature"
that was discovered a little too late to be documented. What you're actually
seeing isn't blood... it's dirt. Red dirt. That particular effect does some
sort of sampling of the texture color at the point where it originates
from... that way, you get yellow sand spray when it hits sand, or brown dirt
when it hits dirt.
In this case, the vent cores are
cherry red from the heat. Those cherry red surfaces are probably (I haven't
checked this) of material type "dirt", the default material type.
The net result is that, when a grenade or rocket detonates against one of
those surfaces, you get a horrifying geyser of blood colored dirt.
on the HBO forum)