Another suggestion for SOv2 shame on me. WHatever!
I figured since a lot of us play with sound off, sometimes on full screen I forget to look up and monitor my health and since i can't hear hits it happens often. So at maybe somewhere between 10-35% health your screen can have an effect added to it, something like this.
(http://i.imgur.com/RKUfl.png) (http://imgur.com/RKUfl)
Personally I don't like that kind of stuff. The way how I handle it in my games is that I make the characters look tired. Black Ops uses that effect you want, but you can barely see anything and it's rather annoying. (btw I don't play BO) Plus the blood isn't coming from the camera.
This isn't a terrible idea, though it would need to be able to be turned off. /effects or something like that would work pretty well.
Doesn't really work outside of a First Person perspective.
Something as simple as a slight flicker of transparency on your character would work just as well.
Sort of a slow transition between the hit transparency and not transparent at all