Try it - if it works use it
I have not had the success with Rune readings that I have now when I incorporated the Blum-Blank Rune. If your Rune Theology says that a Rune has a shape, a meaning, a number, and a sound, then there is no blank Rune because the blank Rune has no sound (I suppose that could be considered the sound of silence - I don't know if the original Rune Masters considered silence a sound though).
As for the blank Rune always being there - that is true, as anyone who has made their own Rune set can attest. When the Rune worker creates her/his own set of Runes, she/he begins with the blank tile upon which the Mystery is engraved.