You can't.

If you're making any artwork, make it so the outlines blend nicely with a variety of backgrounds by testing it against different colors and adjusting as necessary. Death magenta is the ONLY transparent color, near-blends are not accepted, so don't anti-alias against it. Anti-aliasing against one background color is always bad, son.

You can, however use AlphaBlit on your images if you supply a greyscale image that denotes the alpha channel. This is not the same as what you proposed though, and this can only be used with images, not entities, tiles or fonts (unless you make lots of fun little workaround systems with HookRetrace and HookEntityRender and the like).