What you get will depend on the paints you are mixing. Different brands of the same color, even as basic as black and white, have different pigment loads. What you would get from this mix is probably a shade of dark gray. The luster would depend on how much flat base you added—could be anything from dead flat to eggshell, or even satin, depending on the binder and additives in the semigloss. For some glossy paints, killing the shine of the cured coating with a flatting agent is almost impossible.
As others have suggested, if you have a semigloss black, and want it flat, overcoat it with a clear flat. Not only is it more efficient, it's much easier!