you can do nothing, its a limit. That issue have been around nearly since beginning and in years. Its a alpha MULTIPLY issue you property see. SMOOTHSHADING TRUE is just ugly to uses with tiles, so dont do that (im diddent do that my self in both my games due that issue). Yes its might been blocky, but you need to do workaround by other way.
You can do enable and disable SMOOTHSHADING on the same picture, so you can disable it for tiles, but using it for sprites etc.
Im pretty sure this issue will not been fixed, because its property not possible at all. The only way this can been fixed (if possible) is in the CLEARSCREEN command, if its was possible to clear the picture in a different way with a white background and invert the alpha values in its memory (CLEARSCREEN RGB(), ALPHA).