You may have seen some books or websites that mention this "blank rune", calling it the "Wyrd rune" or even "Odin's rune". This itself is a big clue that the author is NOT an authority on runes, and probably has no clue what they're talking about. Whether you're referencing the Poetic or Prose Eddas or any other ancient source of Norse lore, a rune is always a symbol, an abstract idea, a mystery. The rune is the symbol itself, NOT the token or object that it's carved on. Therefore, a "blank rune" is no rune at all, but merely a blank stone or wood tile. And such blank tiles were never used in runic divination by the Norse or other Germanic peoples.
So who invented this fake "blank rune"?
The concept of a blank rune was invented by Ralph Blum, who up until 1982 was a virtually unknown New Age author and UFO novelist of Jewish origins, with absolutely NO prior knowledge of runes or Norse culture, history, or mythology. That year, 1982, was when Blum chanced upon a New-Age goldmine - he claims he got a set of handmade "Elder" Futhark wood tiles in England back in the 1970s, and then ignored them for several years, until one day he found them again and started playing with them. It happened that the set contained a blank 25th tile, probably a spare, in case one of the runes got lost. And not knowing anything about this or any other Futhark, Blum assumed the last tile was some sort of mysterious blank rune with deep mystical meanings, and sorted all 25 tiles into a 5x5 grid, including the blank tile. He proceeded to write a highly watered-down guide to runic divination called The Book of Runes that same year, based on this misguided interpretation.
Blum actually admits he got most of his "rune meanings" not from any authentic Scandinavian or Germanic source, but of all things, from the I-Ching system of Chinese Taoist divination! And it wasn't even the real I-Ching he used, but only the cheesy psycho-babble dime-store translations that were common in magic shops 1982. Nowadays he claims to also be a "cultural anthropologist" and even a "cancer expert" (just try looking him up on the internet, neither claim has any CV to back it up, much less a college degree, let alone an M.D.!) Has he cured cancer yet? Don't hold your breath.
But even ignoring his use of a bogus blank 25th rune, Blum also got the order of the other 24 runes completely wrong as well! He starts the Elder Futhark with Mannaz, which is historically one of the last runes in the series. Had he looked at even one photo of an ancient Futhark inscription, he would have seen the proper order. The sequence always begins with Fehu, NOT Mannaz as Blum claims. Even in the "Younger", Anglo-Saxon, and Armanen rune systems, the first rune is always Fa/Fehu. The sequence runs Fehu-Uruz-Thurisaz-Ansuz-Raidho-Kenaz, which taken in shorthand spells F-U-T-H-A-R-K, hence the name by which the runic alphabet is known. The Mannaz rune doesn't appear until much later in the sequence. A bit like how in modern English we speak of children learning their A-B-C's (after the first letters in the English alphabet), but never their "M-N-O's" because the system doesn't start with M (although unlike modern alphabets, the Futhark were also symbols of magick and spiritual power, not just a plain writing system).
Ignorance is not an excuse once you know better.
In addition, Blum also invented some rather strange new-age meanings for the runes that have ZERO basis in either written or oral Germanic tradition, from claiming that the rune Thurisaz is about contemplation and inaction (when it historically symbolized Thor and the giants, and therefore unflinching and even possibly violent action), to the bizarre allegation that Ansuz refers to the deceiver Loki, rather than being associated with Odin, the God of true and wise speech.
Blum didn't stop there. Apparently the pseudo-rune-literature business proved quite lucrative for him, and since then he's published no less than six other highly misleading books about runes (most of which are basically the typical pop-psychology New Age BS repackaged in a runic cover - his book Serenity Runes even rips off the "serenity prayer" from Alcoholics Anonymous!) And his original misleading Book of Runes is still in print, every 5 years or so a special "anniversary edition" comes out in fancy faux-leather hardcover, with gold-embossed runes all around the edges to give it a deceptive veneer of authority.
It seems any time you take a current "spiritualist" fad and repackage it as "runic" or "Viking" you will inevitably get a bunch of self-proclaimed "witches" and Harry Potter fans buying your book. But while he popularized runes superficially, Blum has, at the very least, caused a huge amount of confusion and done enormous damage to the reputation of runes as an effective and serious magickal system. Now many people pick up one of his books, read it, and think they know all about runes and ancient rune magick, when they are merely on a wild goose chase. And runes - in the imagination of those truly interested in serious magick - end up looking like just another cheesy commercialized fad. Blum doesn't teach you the real meanings of the runes, he teaches a shallow New-Age mockery of them based on his own personal whims.
Real rune rows come in 3's, not 5's.
By contrast, Ralph Blum takes the 24 "Elder" Futhark runes, adds in his "blank rune" for a total of 25, and then divides them up into 5 groupings containing 5 runes each. Not only is this totally un-historical, it also completely divorces the runes from one of the fundamental beliefs in Germanic cosmology - that everything in life, every cycle, every process, is three-phased. There are three phases in everything: Arising, Being, and Passing Away towards a new Arising. Hence why there are also three Norns, or Goddesses of "fate" in Norse/Germanic cosmology: Urðr (that which Was), Verðandi (that which is Becoming) and Skuld (that which May Become). There were three co-equal classes in ancient proto-Germanic society (farmers, warriors, and priests/mystics). And so on. Not a single mention of "fives" or even of the number five having any sacred meaning in Germanic Lore!
If on the other hand you want to practice real, dynamic and powerful Rune Magick like the revered rune masters and skalds of old, then whatever rune system you use, do NOT use a blank tile for rune casting. If you have a rune set that includes it, keep it locked away as a spare, to carve in case you lose one of the actual rune tiles. But if your rune set came with one of those horribly misleading books by Ralph Blum or other fad authors, chuck the book in the trash, you're better off not reading it, so you won't have to waste time unlearning all of its wrong information and false meanings.
There are much better books on Runes and Rune Magick out there, look here for a brief list.