The problem with blood (and most liquids) is their inherent transparency. To look real (as most people perceive it) it's deep red (boardering on purplish tones) with almost black in the thicker areas. On a black background it gets lost, especially considering the Chuck print process seems to have a certain amount of transparency in the printing. Punch up the red to make it appropriately visible on black and it starts to look more like red acrylic paint.
Here's a quickie of the same splatters on a white background:
Ignore the button on this image (forgot to turn the damn layer off!)
The white makes the blood POP more.