Well, I'm not going to go into creating the art assets for "blood", nor will I go into details about the specific values you should use for the particle emitter (partially because there are just too many, and partially because it's more to your own personal tastes than anything), but I can outline one technique you can use.

Create a new Particle System game object. Set its effect to whatever you think looks best for "blood" (be sure to check the "One Shot" option, for instance). Then, when you want the blood to emit, move the Particle game object to the position where you want the blood to appear with some code (usually the hit location of a collision), and call Emit().

I would adapt the code from the FPS tutoiral that makes sparks come out of the wall. Then create the appropriate particle effect, then right an IF statement into whichever script that controls the creation of the particle effects that says if the object the ray cast just hit is the enemy create blood particles, else just create sparks.