Right you are Nick.... it is indeed blue, but it wasn't always that way... it was originally green..... folks have chemically treated 1928-1953 red seal $2 notes to make them appear yellow, earlier (circa 1870s) red seal $1 notes have turned to a very dark blue to nearly black through oxidation.... Sometimes the green backs of modern notes are a light blue (as are the seal and serials of this note)..... I have heard a few possible causes, such as prolonged exposure to sunlight, chemical treatment, etc., but I really don't know what caused this, other than it didn't happen at the BEP.
No amq, it is neither common or an error..... I would venture a guess that it holds the normal premium on this note to a minimum..... (unless you find just the right person on just the right day). Whether naturally or artificially, the note has been altered.