Agree with Camron. If B1's forward motion with her arms - even if it was from -25 degrees to straight up - causes contact which interfered with A1's shot, it sounds like a foul to me. B1 is initiating the contact in question - I've got a foul.
Same as when the defender has hands straight up, then pikes the arms down on the shooter, then goes straight up again - and gives you the shocked look when you call the foul