Admittedly, I don't know much about the game aside from what I saw in GameSpot's E3 video. But I couldn't help but notice that all the xenomorphs that were being shot up had regular ol' red blood. I thought they had green, acid-like blood.
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it's easier to notice when the Xenomorph isn't right on top of he guy hitting him. You can see when he shoots a few with the pulse rifle they bleed good old green acid. It just isn't a lot of blood coming out of them until they die.
Playing: Black Ops 2, Guild Wars 2, Halo 4 Anticipating: Gears of War: Judgement, GoW: Ascension, Aliens: Colonial Marines
I remember seeing an earlier build of the game where the acid was barely noticeable and they were still using hit markers. It's much brighter and obvious now. http://www.youtube.com/watch?v=g0ChHiN6ndQ