If Disney's Alien Encounter was able to imply that someone was walking on a catwalk above us, and then scream, and then hit us with a water effect - making us THINK it was blood - I don't see why Universal wouldn't be able to imply something similar.
Imagine someone "off-screen/out-of-view" that the werewolf jumps at, we turn to look into the darkness he just jumped into, get splashed, and then we cut to the werewolf holding the remains of someone/something minus the literal viscera/blood red color(for rating purposes).
Could also always do the Evil Dead trick and make the person he jumps at "undead" or other-wise and have the blood be black/green - cause then you get past the rating boards because it's NOT red.