SDL: Blit to a non-visible surface

[IllegalCharacter]

Since your masks are all zero, it filters out any colours you're trying to put. Hence why you have black

Try putting this in:

    Uint32 rmask, gmask, bmask, amask;
    rmask = 0x000000ff;
    gmask = 0x0000ff00;
    bmask = 0x00ff0000;
    amask = 0xff000000;
    
    SDL_Surface * output = SDL_CreateRGBSurface(SDL_SWSURFACE, 100, 25, 32,
        rmask, gmask, bmask, amask);