Heh, thanks for the advice!
The blood splatters were actually photos of stars from an astronomy website, with selective color replacement added. I increased the contrast on the photos and used the magic wand tool to select the shadow areas, which I then clipped out, allowing the red star base layer to show through. Nothing fancy, really.
I initially considered using a motion blur on the stars but it looked terrible, so I dropped the idea of implying motion in the shadows.