I had an idea in regards to Jacob and the Black shirt guy.
I think they are the same person but BSG is an older version of Jacob.
Jacob did something in the future that may Doom humanity. Which causes him to go back in time to stop himself from doing so.
The reason BSG can't kill jacob is because he is an older jacob and by killing his younger self he stops the end of humanity. But he needs a loop hole as he can't kill him younger self.
Only problem is due to course correction one of our losties will end up taking over the jacob role and the loop continues...

You know what? This is so frustrating, because I just had this idea that maybe the reason we hadn't learned Blackshirt's name was because it was "Jacob". That they are the same guy, but the nameless one is older, and suddenly the "loophole" stuff made total sense because he can't kill himself in the past. I came here specifically to start my very first theory thread and you beat me to it. Damn. You've even thought it through better than I had.

So yeah, I like this idea. It fits with the yin & yang stuff they've been feeding us all the way through the show - two sides of the same coin, one can't exist without the other - but also works in a commonly understood sci-fi way, a refinement of the Grandfather paradox.

There's a Doctor Who story with a similar theme, the plot of which I should spoiler font for those who haven't seen it:

In The Trial of a Time Lord (1986), the Doctor is put on trial for some nebulous crime or other, involving interference in the affairs of the universe. It's a much-maligned story and not without good reason, but it does feature one of the best ideas ever to grace the series: the main villain (the trial's prosecutor) eventually turns out to be the Doctor himself - more specifically an evil future version of himself - who wants to steal his own remaining (past) life in order to extend his own (future) life. Yes, it's bizarre, but it's a bloody interesting idea. Sadly the writer died before completing the story and the whole production fell into chaos, so it never gets explored properly.

Dunno, maybe it's too hokey. Maybe it's better if they're separate beings. But it would definitely explain why MIB would have to go to a hell of a lot of trouble to work out how to kill Jacob. It doesn't quite explain how MIB can somehow kill his past self by taking over someone else's body, or by convincing someone else to kill him, or why they refer to each other as if they're separate people.

BUT in the spirit of "any idea is possible until we see the Season 6 premiere, and it will probably all be scuppered tomorrow when they screen the first episode on that beach in Hawaii and the internet is full of spoilers" I'd say Jacob and Not-Jacob being the same person is as good as any other theory, and the idea that future Jacob has a good reason for killing himself (to avert a future catastrophe) makes perfect sense.

But wouldn't that cause the ultimate paradox? Ben (or anyone, for that matter) killing his younger self still means that the older self ceases to exist...which means he can't come back to kill his younger self.

Yes, the ultimate paradox. So he'd need a very, very clever loophole to get around it. Not sure how pretending to be Locke does the trick, but "the ultimate paradox" sounds very appealing.

I actually think the Loophole is much simplier. There are rules that both Jacob and MIB have to abide by, one of them being that they cannot kill each other and they can not have one of their followers kill the other.

The Loophole, is that nothing prevents one of their own follwers (Ben a follower of Jacob) to kill his own leader. So MIB, has taken over Christian's body in order to trick Ben into leaving the Island, telling Locke he must die, putting Ben in the position of killing Locke and then returning him to the Island. All so he could take over Locke's body and convince Ben to kill Jacob.