SPOILERS of course for Batman The Cult.
Blackfire is one of those character you have to see to get the full impression of but I'll try to explain the impact the mini series had. Stories like No Mans' Land get a lot of attention for making Gotham into a hellhole, Morrison and Snyder gets credit for stories that deal with Batman losing his mind due to drugs/conditioning. But this is the story that threw all those things together before them. I will admit I grew to like The Cult the more I read it and really got the scope of what was being done. That said it is a little dated in places (like Batmans' vision of what a pimp looks like.)
Deacon Blackfire is a man that Batman writes off as a kindly soul that looks out for those in need. He realizes too late that he's responsible for the missing homeless people as he's brainwashing them into being his personal army to kill off anyone he chooses. Their ruthless, Gotham is in a state of panic as people are dropping like flies and anyone that opposes him is murdered or has attempts on their lives. What's worse is Blackfire has a good portion of the city backing him for "cleaning up the streets." Long before Bane Batman is broken--in spirit, mind and partly in body--thanks to weeks of drugs and lack of food. He doesn't know what's real and starts to buy into the act himself. In fact he's so gone in one scene he thinks he sees Two Face and attacks with a gun. Only to snap out of it after he pulled the trigger not seeming to realize that he may of killed a man.
To say the Deacon isn't who he first appears to be is a understatement. Batmans' first impression is the only time we see him as saintly since he is front and center throughout this brainwashing people and causing chaos. He uses religion to get what he wants knowing the affect it as on others. But one of the most fascinating things about him? Blackfire claims to be an immortal and we never know if it's true or not. Not even Gordon can disprove it since they found records going back further than they should. Of course it'd be easy to say he was crazy too because he certainly does things that fit the bill in that regard. The means he goes through to achieve his "immortality" is actually one of the most disturbing scenes.
Blackfire manages to break Batman in another regard as the dark knight flees Gotham not wanting to return then becomes so obsessed with revenge that he gives little concern to the lives of others. He leaves a woman to be ripped apart because he can't be bothered to be slowed down from his mission. Even leaving his Robin to fend for himself well aware that he might be leaving him to die. While Batman ultimately decides against killing the man that broke him it's not because of his morals. It's because Blackfire wants to die to become a martyr. The brutal beat down Batman gives is designed to humiliate him in front of his bloodthirsty followers. Once he does Blackfires' hold on them breaks leading to the mob rushing forward to tear him apart. Jason tries to save him only to be held back by Batman who claims it's too late. (*1)
It's one of the stories where Batman is truly messed up, showed Jason as a good Robin and is mentioned in Death in the Family as a reason for Bruce being off his game. Gotham was taken over, the government couldn't overcome the mob and most of it was mass chaos. I kind of hope Blackfire is the big bad at the end because it makes sense and would be something different from the standard rehash of bad guys.
I'm guessing this is going to be a reboot of the character which is a shame as I think this would work better with The Cult intact. Gordon is taken out first in Batman Eternal, something Blackfire would know would cripple the city as he saw the threat Gordon posed before. If that's him that's taunting Bruce at the end I could see it still working since he could have unmasked him during any part of Batmans' capture. His death wouldn't matter if they chose to explore him having actual powers and I think it might be a possible connection for why Jason returned from the dead too. Snyder seems intent on discussing how Gotham is a character of it's own and Blackfire certainly fits the bill for someone that could corrupt it. This is a man that knows how to manipulate others and broke Batman. That's a scary thought especially if he truly does have powers. If written right he could truly become one of the biggest threats in Gotham--he already was before his death but this time he might stick around longer. That's a disturbing thought.
*1 In case you were wondering Jason is actually holding a tranquilizer gun. They have been taking down the cult with them for most of their fight.