sadmenu is a program (
heavily based on mostly Marcan's advice and Menuloader) that loads the System Menu, but patches it in memory to use "zcon.bin", "zanner.bin", and "zound.bin" instead of the normal versions. Therefore, when you load it, all the banners will be black. Just press "reset" to go back to the normal System Menu.
It should be possible to edit the U8 to add extra files with the appropriate names. In that way, you could test a new banner without fear of bricking! Although, I haven't actually tested that (because, my laptop defunct, I can't even use wiifuse properly)... for one thing, make sure your U8 is good.
This is also useful if you have any other pressing reason to patch the System Menu.