Bytes 0xD851-0xD853 of tileset.exe are responsible for storing the transparency color so you can modify those to 0x000000 in order to get black backgrounds by default. This won't help much with your mask problem because now both transparent and opaque pixels will be represented by black, but hey, you can always have another copy of tileset.exe that sets transparency to white (0xFFFFFF) instead and then invert colors in your "simplest graphical editor". You can also invest some time in a search for a more complex graphical editor. Each option should be faster than waiting for a return of Neobeo, whose last activity was in January 2014.