Blood is gross, but if you insist, I recommend doing it in-camera so you cannot ruin it with MS paint or have to rely on effects. Try clay, or a piece off saran plastic wrap colored red. Really red. You'll have to use a red sharpie and go to town on that saran wrap, but it should probably look good if it's thoroughly colored.

I cannot help you with an effects program, because I have not found one myself. I use Blender, though, which is more of a 3D modeling software where you have to do everything manually and there are not may presets like smoke and fire and such.

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"Whatever you do, do all to the glory of God." - 1 Corinthians 10:31b

Dittos to the above two.  I prefer my bloodless action flicks, but do it in camera.  The worst thing you can do is use a cgi splatter effect.  I'd go for clay, mixing red with some darker color, probably some black.  I haven't mixed clay in some years.

My sister is pretty good at special effects makeup. She made blood once with ketchup, food coloring and corn syrup. Never again, stuff smelled awful under the lights. So don't do that.