Please *Register* To see more great tutorial and request Great Graphics!When creating blood always be aware of the background. I'll show you how by changing the color it has its influence on the end result of your blood-effect.
I made a black-grey gradient from top left to right bottom.
The darker parts will end up having some dried blood-effect, while the lighter colors will look as if the blood is fresh.

I chose a brush that's provided with PS, namely Spatter, 59 pixels. In order to have some randomness i went to the brush preset and changed some things. I added shape dynamics (angle jitter, size jitter and roundness jitter, all to 100%) and scattering (scatter to 142%) but play around with these settings to find your spatter.
The color I chose is #892121. When you double click on the foreground-color in your toolbar, you'll get the colorpicker. At the bottom right side there is a #. Next to it you can input the number given above.
With opacity to 52% and flow to 55% I brushed around a bit.. went over some parts twice to get some colorvariation.

Alright .. a very easy step.. just duplicate the previously made layer. You can do that by dragging the ground layer over the icon 'create a new layer' on the bottom of your layers-pallette.

On the top you'll see a box with different blending modes, choose multiply.
Looks better but not real yet, right?
You're close to finishing..
Make a new set (the icon on the bottom of your layerspallette, that looks like a folder) and drag the two previously made layers inside of it. (make sure you keep the right order of them the one that has as blending mode 'mulitply' on top of the ground layer)
This set can also switch from blending mode. You can find that box at the same place as for the layer. Set it to color burn.

Now you'll see that where the background is darker you get dried blood and where it's lighter fresh blood.
Now just experiment a bit, have fun!!