there are two reasons i thot of at the time.

1)people who hadn't played the game and just came because they had heard there was a final fantasy concert in town might have not expected that intense scene, especially during sucha sad song. than fans. thats how i understood it. especially cuz the "black screen" only showed for about a brief second.

2) another thing that i thot might've happend was a slight technical difficulty. from the beginning to the intermission of the concert, there were speratic(?) technically difficulties, like the occasional "popping" of the speakers, microphones going out, etc.