The simple answer to the latter question in my view is no, but the real answer is more complex than that and has to do with historical facts, which I can explain by addressing the other two questions.
Is the blank Rune a thing? Yes and no. Yes the blank Rune is a thing, but only since the 1970s at the earliest. Is the blank Rune a thing in the Elder Futhark or any of the other furtharks? No. There is no evidence in the eddas, sagas or any other relative historical documents that even suggest that such a thing existed.
The first mention of a blank Rune comes in Ralph Blume's 1982, "The Book of Runes". This is where the controversy around the blank Rune begins. So, let's talk about Blum's take on it.
Blum refers to the blank Rune as "the unknowable", "the Divine, Odin, the Allfather". Both of these descriptions are entirely inaccurate.
The unknowable. The whole point of Odin sacrificing himself to himself was to gain the knowledge of the Runes. So, the idea that there would be a Rune that represents the unknowable goes against Odin's actions.
|Old Icelandic Rune Poem for Óss|
I won't even venture into his detailed explanation of the meaning of the blank Rune, where he gives no less than eight different things that it represents. It further demonstrates his lack of understanding of the cultural history in which the original use of Runes formed.
For those of you who aren't so concerned about the blank Rune's complete disregard for the historical and mythological contexts, consider this simple point. The Runes are an alphabet. The term "Futhark" is literally the word formed by the first six letters:
So, yes the blank Rune is a thing, a very recent thing. Should it be used when seeking guidance from the Runes as an oracle? No.