The blank fire rune is a quest item used in The Slug Menace quest. It is made by using a chisel on a normal rune essence or pure essence after adding the missing Page 1, Page 2 and Page 3 back into Maledict's holy book. During this process the player must also make a blank air rune, a blank earth rune, a blank mind rune, and a blank water rune.
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.
When used on a Fire altar in the usual way it turns into a special fire rune which is needed later in the quest.