Before the theme for season 3 was announced, the predominant theory was that it would be about witches. Fans referred to it as "AHS: Salem". Now of course the subtitle ended up being "Coven". But was the theory wrong? Not really. So what if the name isn't right? It's just what we're calling it so others know what we're talking about. You're absolutely right that it wouldn't be subtitled "Blackout" if that's what the season ended up being, because the subtitles always describe the institution. The name is irrelevant to the theory, and as such I don't see why the name we give it in discussions on this sub has any effect on its validity or potential.
To be fair, this theory did have origins long before the profile pictures changed a few days ago. Right after Paleyfest, I made a post referring to season 6 as a "blackout season". I meant this metaphorically since Ryan is "keeping us in the dark" about what this season is about, and so I was just suggesting how maybe we won't know much about it going into it (and look at where we are now). A lot of people took that literally to mean that this season would be about a blackout. I don't think anyone really took the theme seriously back on that thread, and just offered it up as a fun idea to toss around. But the black profile pictures have people guessing that maybe this all is making sense. Personally I don't think there's much meaning to the black pictures, but we all know Ryan loves to work in mysterious ways, and he would totally be the person to both keep us in the dark and surround us with black squares to hint at a season in the darkness.
As for what the actual storyline would be about, maybe it's about cave dwellers, or maybe it's about the Amish without electricity. Maybe there's a power outage in the first episode, and during that brief amount of time, people go missing or people are brutally murdered. The lights come back on an hour later, and the rest of the season follows the aftermath of the blackout. Who knows? I honestly could see a lot of potential with it. Either way, we'll know in just a few days what the actual theme will be, so let's not get all worked up about all these theories this late in the game.