As someone who has actually performed on live stages of varying sizes hundreds of times, I'm uniquely qualified to address this issue, and along the way we're going to crush one of the greatest myths that is (cleverly) perpetrated by the entire k-pop industry. Gosh.
Firstly, let's take a look at some footage from the live concert that the poster is referring to - the notorious 2008 Dream Concert, where fans of certain other k-pop groups controversially (don't laugh) decided to "black ocean" or shut off their lightsticks while SNSD performed, for some bullshit reason nobody is even completely sure of and that even the notoriously egocentric-in-public-statements Super Junior didn't think was all that important.
The crowd did still cheer... somewhat. They didn't throw anything or hurl abuse, which made them a lot more polite than certain other audiences. However, all except a tiny corner of SNSD fans (seen in the video at 2:09) turned their lightsticks off. How could SNSD even perform to that at all without crying or getting pissed off at the crowd? Are SNSD super-strong angels? Are they robots? Or do they just not give a fuck?
Well, to get the true answer to this question, in typical Kpopalypse style, we're going to go on a massive tangent and talk about something completely different... yet in a way, very similar.
Meet Darrell Abbott AKA "Dimebag Darrell", former lead guitarist of the American heavy metal group Pantera. Dimebag had a long run with Pantera, forming the band with his brother in 1981, but the group didn't see commercial success or become influential in the global heavy metal scene until they ditched their ludicrously shithouse early glam rock style in favour of a much more refined and skilfully executed thrash/groove metal approach.
After hitting a creative peak in the early 90s and releasing a string of successful albums, Pantera broke up in 2003 when their lead singer Phil Anselmo decided for whatever reason that he wasn't going to return to the group after a hiatus (obviously a bully victim because that's the only reason why groups ever lose members isn't that right k-netz). Having already achieved great respect in heavy metal and guitar aficionado circles, Dimebag then went and started again from scratch, forming a new group with his brother called Damageplan.
Then at one of their first ever shows, a crazy fan walked onstage with a gun and shot him dead. And you thought EXOtics were insane.
I remember when I first heard about the shooting, it was only a few minutes before I was about to go on a stage myself - not a good feeling. Dimebag's fate was actually a really scary moment for almost everybody I knew working in the live music scene at the time, because it highlighted something that we all preferred not to think about - how vulnerable performers are on a live stage. We all knew that Dimebag was a sitting duck - anybody could easily do what happened to him or something like it to any one of us, at any time, and there is nothing any of us could do to stop it.
So why are performers so vulnerable on live stages, and what's this got to do with black oceans and SNSD? The answer lies in this screencap of the Black Ocean concert:
What do you see in this picture, besides SNSD? That's right - three big-ass spotlights pointed right at the faces of the girls. That's so you can see the girls doing their thing on the stage even if you're 10 or 50 or 100 or 200 metres away, and those spotlights are on almost the whole goddamn time. Oh and look, there's two more spotlights off to the side. You can bet it's a symmetrical layout and there's even more spotlights on the other side too, so that makes seven spotlights covering the girls from multiple angles, and that's assuming that we're not missing some other extra spotlights that are out of frame.
Now guess what - when someone shines a big torch in your face, and you look right at it, you're blinded. You can't see a fucking thing in that direction. Now imagine that the torch isn't just a torch but an uber-powerful spotlight like the ones that they use at concerts, that are several times more powerful than car headlights. Now multiply that brightness depending on the size of the venue and the amount of lights used. Then add the extra effect of any other lights in the lighting rig such as dimmer "floodlights" which are often used by the dozens at big events. At a daylight concert it's different, but any big gig during the nighttime and the performers can't see past the first few rows thanks to all that shit shining in their face. I can't remember a single smaller concert I ever played at night with proper lighting where I could see more than three rows into the audience from the stage, and on very large stages where the very front row of fans isn't even anywhere near the fucking stage... well, you can't see ANYONE. Look at the photo again, and how fucking far back from the stage the first row of fans is. You could park a small aircraft in that fucking gap. Think they can even see that far from that position, think again.
Those bright lights are a big part of the reason why that gunman was easily able to walk right up to Dimebag and shoot him, no problem at all. Poor old Dime wouldn't have even seen the guy approaching the stage, he would have been blind as a bat looking out into the audience (and yes he did come from the side but he still would have had to approach from the front to get there). Performers can't see a goddamn out there, even if the crowd are all holding one of these bad boys.
Now, if some dude can just stroll up onto a stage with a drawn pistol and shoot someone without anyone really registering what's happening until it's too late because they're all blinded like a deer in headlights, do you actually think that someone's going to notice a bunch of people a hundred metres away waving (or not waving) lightsticks? They'd be just as likely to hear your mobile phone ringing in your pocket over the sound of the PA system. Of course, they might notice your lightstick if it's a gig where there's gangways and they get to walk out into the audience a bit, or if they suddenly bring the house lights up, but not if it's a standard stage setup with you doing your two songs in a home gym sized stage area that's completely floodlit and spotlit and them fucking off. Between being completely blinded as soon as they look forward, plus having to sing and remember all the dance routines in such a compressed timeframe... SNSD probably didn't even know that they were being "black oceaned" at all, and likely found out what happened after they got offstage and their backstage handlers told them.
On a related note, yes this means that the notorious "waving and smiling at a random audience member" is kinda bullshit too. If you've ever had that happen to you at a k-pop concert, and it was at night or indoors, and you weren't in the first three rows, guess what? The performer didn't even fucking see you, she just waved in a random direction and gave a friendly smile, because that's what she's trained to do and she knows there's probably some fans in that direction who all thought "OMG SHE SAW ME!!!!!1!1!". These girls and guys in k-pop groups are probably even told to try and evenly distribute their fanservicey gazing across the whole venue, so that it doesn't matter where you're standing, your favourite guy or girl is going to look kinda in your direction eventually, and confirmation bias will do the rest.
And that's why SNSD were cool as a cucumber about their black ocean (until later when they presumably cried a bit or however these groups are paid to react). It's also probably why Crayon Pop's Choa only very narrowly escaped getting her tits groped in public by a crazy fan at a club event recently.
Bright lights in your face affect things like distance judgement, as well as molester judgement. Good thing about those helmets. Stay safe like Crayon Pop, kids.