I wanted to put a new little theory out there that's been forming as to the nature of smokey. I don't have a theory on what the black smoke is or how he got that power or whatever, but more about what he does.
We've been getting these hints, even calling it Cerberus, that the black smoke is some kind of judge of character. Maybe he is, maybe he isn't, but what if he is? He scans people, and it is that scanning that may tell him whether they are good or bad.
For example, on the Black Rock the officers had all kinds of bad things going for them, dealing in slaves, etc. One of them even started killing everybody. All the slaves were probably killers... except Richard was different. That killing was an accident, so MiB didn't kill him right away. Maybe that's why MiB hasn't kill him for the last 140 years. He's really a good guy.
And now we have Jacob bringing people to the island, trying to prove to MiB that man can do right and not just "destroy and corrupt". But, where is Jacob when he brings people to the island? The Black Rock came in on a storm and ended up in the middle of the jungle, a good hike away from Jacob. So how would Jacob even know that he could say "See, here is a good man"? Unless, of course, he has some mystical way we don't know yet or it has to be proved over an extended period of time. But....!
Maybe Jacob doesn't even need to be there to prove it. Jacob can bring the people to the island, and Smokey judges them. When smokey scans them he can tell how good or bad they are, perhaps. Jacob is trying to bring the person that will finally prove it to smokey in one of his scans?
If that's the case, the question is what is the nature of smokey? Perhaps as he said he was a man once, somehow he became smokey, but what if embedded in smokey is some kind of code? Like computer code, but it doesn't have to be. But something that governs his behavior. If he finally scans a person "good enough" or whatever, he may shut down, or be convinced to stay, or whatever.
Okay, so lots of "somehow"s and "whatever"s in there, it's just a working idea.