Pens such as these invisible ink pens (amazon.co.uk) can write in invisible ink that can only be read when using a UV light. A nice way to do this clue is to give the hunters the UV light earlier on in the treasure hunt and then later on give them a clue that is a piece of paper that looks blank but actually contains the clue in invisible ink. The treasure hunters must realise that the light they found earlier can be used to reveal the clue.
You can make this more difficult by writing over the top of visible text - the hunters will be mislead by the visible text, but make sure they realise about the blacklight in the end!
For an alternative to this see our Invisible Lemon Juice Ink Writing clue