There are actually 2 ways to accomplish this, one through procedural shaders and the other through straight textures.  Since the OP is not very familiar with the Material Room, we will deal with it with an image and texture mask.  Let's take it back to basics, starting in photoshop with a PSD image of blood.  My selection is deviney's Rons blood which is in PSD format at a resolution of 2208x1948.  A fairly large file.