A scroll of blank paper, cost 60, is always an 'unlabeled scroll' and is always blank. They may be obtained normally or by blanking other scrolls with a wand of cancellation or by dipping in water, intentionally or otherwise. Killing a paper golem will also often yield one or more scrolls of blank paper.
The main use of scrolls of blank paper is to write new scrolls with a magic marker. Attempting to write a scroll of blank paper with a magic marker will produce the message "You can't write that! It's obscene!"