Yes, but how do I compute the resultant color?
r g and b * alpha ? (didn't try, but I think its wrong...)

I tested against pixa > 5, so I got most of the blood without the blackness, but I do have some very dark spots, that should have blended with the terrain...

I guess I have to somehow take the terrain's pixel and add the blood pixel times its alpha to the terrain pixel, right? But, it might result in values above 255?