This is the climax from John Kember's "Happy Feet" for solo piano (annotations are mine):
On the final beat of this excerpt, there's an octave interval (#2, #3) in the right hand, with an acciaccatura (#1) before the lower note.
Both the acciaccatura (#1) and the upper note of the octave (#3) are B flat. My question is: does the flat accidental before the acciaccatura (#1) also apply to the main note (#2)? In other words: is the main note (#2) played as B flat or as B natural?
Arguments in favor of B flat (i. e., the accidental applies to the main note as well, the interval is a perfect octave):
- Normally, accidentals remain in effect for the remainder of the measure.
- B natural isn't part of the indicated chord, D7/♭9/♭11/♭13, whereas B flat is the ♭13 of D.
Arguments in favor of B natural (i. e., the accidental does not apply to the main note, the interval is a diminished octave):
- An acciaccatura is normally a tone or semitone above or below the main note.
- If it was also a B flat, what would the acciaccatura even indicate? It could be an arpeggio, but then I would have expected it to be written with a wavy vertical line instead.
This answer says that an accidental on an ornament (like on a regular note) remains in effect for the remainder of that measure; but the ornament in question is an appoggiatura, not an acciaccatura (also written as a grace note, but without the oblique stroke through the stem), and the answer doesn't make it explicit whether it applies to "all appoggiaturas" or "all ornaments, regardless of their type".
In addition to an answer to the question which of the options is correct, it would also be interesting to understand why the arguments in favor of the other option do not apply.