Duh.
I mean no one even knew about the secret deity "Kraft Macaroni & Cheese" until now....
ahh, crap. I let it slip. Goshdangit...
Ha ha. No. This is not true. Do not worry about Cairn fellow human Planeshifters. He has gone away for awhile. There is no such thing as KRAFT LORD OF MAC THE NINTH LEVEL OF CHEESEDOM. Anyone who feels like discussing the fictional KRAFT LORD OF MAC THE NINTH LEVEL OF CHEESEDOM is encouraged to join us crazy people in the dark cellar beneath the abandoned church looming terribly over the crowded streets of Arkham Massachusetts. Let us go. Do not bring weapons.
I treat the Black Flame in the same way people view the idea of a vast alien conspiracy. I feel as though there are so many dead ends and crazies out there who would make the entire idea of looking to closely into the Black Flame so much of a social faux pas that it would give the investigator the nut job label. Anyone who did get to close to uncovering the truth would likely disappear under mysterious circumstances. Those who do learn too much might even be driven mad, and become Black Flame worshipers themselves.
I would compare our modern world, with its cameras and records everywhere, to a fantasy world with accessible scrying. Even with access to information, the real truth would be so mired in crack pot theories and twisted folk tales that getting anywhere near to the truth would prove to be nearly impossible. If we can assume that Black Flame worshipers are actively spreading misinformation as well, we can safely say that it would be very easy for an investigator to fall down a rabbit hole that is filled with entirely fictional information.
Even if some one where to successfully scry on the Black Flame itself, there is reason to believe that the Flame would have the ability to warp what the diviner was seeing, thus providing even more misinformation. The Black Flame would not think like any mortal. It would think in terms of centuries, and eons, not in seconds and minutes. It's plans are unfathomable, and even its worshipers do not fully understand what it wants. What makes the Black Flame so interesting is that there is so much mystery and deception going on. The gods themselves probably don't even understand it's motivations.