It might be used to generate texture variation through alpha manipulation. Basically, the area could have red or (dark) brown applied to it, giving the texture either a head bruise or just filling in the area with the same colour as the surrounding hair. If that's the case, you should be able to see the same blob on whichever texture they're using to handle these variables.
Valve experimented a lot with stuff like this in l4d2, to let the game generate distinctive variations of the infected, without needing thousands of different textures. They're using it quite a bit in csgo now, too. Most newer models have a mostly-white texture that you can assign a colour in Hammer.