First, you need to isolate the actor (foreground) from the background. Then, when you’ve done that, you put the layer that just includes the actor over the blood splatter layer, which is on top of the background layer.
You will probably need to use rotoscoping (frame-by-frame animation of masks) to isolate the foreground from the background.
This is basic compositing. See this page for some introductory material about compositing.
Perhaps you should begin here:
https://blogs.adobe.com/toddkopriva/2010/01/getting-started-with-after-eff.html
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Todd Kopriva, Adobe Systems Incorporated
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