The blank water rune is a quest item used in The Slug Menace. It is made by right-clicking on Page 2 of Brother Maledict's holy book and choosing the "shape water" option with a rune essence or pure essence in the inventory. During this process the player must also make a blank air rune, a blank earth rune, a blank fire rune, and a blank mind rune using page 1 and page 3. The player needs a chisel in their inventory to make these runes.
Make more than one blank because you can fail while chiseling the shape of the rune and/or binding it with the element of the rune.