Brush Tool Blending Modes
When using the brush tool you can change the Blending Mode of the brush (in the Option’s bar) to get different painting effects.
For example, a good way of creating digital blood is to select red and change the brush’s Blending Mode to Multiply.
You can then start painting in a new layer, once you go over an area you have already painted on, the red will get darker with every pass, giving it the illusion of blood.