blank paper

If it shows up under a blacklight you can just use a blacklight while you write

If the invisible ink is truly invisible under any conditions, it is useless. Therefore, if you were to write in invisible ink, it would have no point, as it is truly invisible. There then would be no reason to write with it, and you would write with normal ink, because, yes, you would never know when you are out with invisible ink. Time to get out of the shower.

Obviously the concept of "invisible ink" is that it is invisible under normal lighting, not under "any condition".

Yeah...