As you expected...
    the code is a simple modification of the 4-way connected flood fill.

  public void floodFill8(int x, int y, int fill, int old) {
      if ((x < 0) || (x >= raster.width)) return;
      if ((y < 0) || (y >= raster.height)) return;
      if (raster.getPixel(x, y) == old) {
          raster.setPixel(fill, x, y);
          floodFill8(x+1, y, fill, old);
          floodFill8(x, y+1, fill, old);
          floodFill8(x-1, y, fill, old);
          floodFill8(x, y-1, fill, old);
          floodFill8(x+1, y+1, fill, old);
          floodFill8(x-1, y+1, fill, old);
          floodFill8(x-1, y-1, fill, old);
          floodFill8(x+1, y-1, fill, old);
      }
  }