That's a trick question. An "alpha channel" is really just one channel that has no "color" of its own. It's just transparency information.
A:M will render the color black around an object in an image that has an alpha channel, but that black is really in the color channels just like the image of the object.
you could load your targa into Photoshop, load the alpha channel as a "selection", invert the selection, and fill the selection with whatever color you need. I don't know if this will creat a halo problem or not.
If you need it to be black, why do you want to change it from black?
If you just need the object rendered on some solid color, turn the Alpha buffer OFF and set the camera background color to whateveryou want.