Unfortunately, I can't seem to escape color-additive alpha blending. For example... if I take a completely black square (no colorkey) and alpha-blit it to the background, I want a "tinted-window" effect, where it simply darkens the pixels. Instead I get nothing, as if it can only make the bg *brighter*, never darker.
My hunch is that this has something to do with the rendering state... specifically, the blend factors for the source and destination (XXX_ZERO or XXX_ONE).
Can someone please explain to me how these work, and what things I should set to get that nifty "tinted window" effect?