The chairman of the Better Together campaign said voters must know what would happen if, after a Yes vote, the rump UK Government rejected Alex Salmond's plan to share the pound in a currency union.
Chancellor George Osborne has called such a sterling zone "unlikely", though the SNP say this is scaremongering.
Speaking to the Sunday Herald, Darling said November's White Paper had to explain what would happen to pensions, the national debt, and above all the currency in the event of a Yes next year.
"If they don't come up with answers, the thing is going to fail," he said. "They have gone from favouring the euro, to using sterling in the same way Panama uses the US dollar, to saying we'll have a currency union.
"But with a currency union … the other side have to go into it, and that is doubtful at best. And if you don't have a currency union, what's Plan B?
"Will they just use sterling, will we have our own currency, will they go into the euro? Let's see."
Like Alex Salmond, the former Labour Chancellor seems obsessed with a TV debate on the referendum. But while the First Minister wants a square go with David Cameron (see previous page), Darling wants the First Minister to step into the ring with him.
He wrote to Salmond on Friday on the issue. He may have a long wait for a reply. "We're not reacting to backbench MP Darling," sniffed one spin doctor.
But Darling is adamant that he, not the PM, should be the one to meet Salmond in verbal combat.
"I'm ready and willing to debate all these key issues with him, whenever he wants, as often as he wants.
"I cannot understand his refusal, except that I'm beginning to think that he's running scared.
"David Cameron doesn't have a vote in Scotland. I've got a vote in Scotland. Alex Salmond has a vote in Scotland."
It sounds logical, but Better Together's biggest donor, Ian Taylor, the Tory-supporting boss of energy trading firm Vitol, doesn't have a vote in Scotland either. So residency isn't everything.
In truth, Darling is Cameron's human shield, because Salmond, whose motives are equally transparent, would love the referendum to be a straight choice between himself and the Tories.
Welfare cuts, the bedroom tax, tuition fees, Royal Mail privatisation, you name it - Salmond would throw the lot at Cameron in a TV debate, and Darling and his Better Together team know it.
So the PM is off the menu, and instead cool, calm Mr I-Saved-RBS is all that's on offer.
Darling would certainly be a match for Salmond. That silky, rapid-fire delivery packs an extraordinary amount of venom.
In a few quick sentences he suggests Salmond is "very status conscious", "has a very high opinion of his ability", and secretly pulls all the strings at Yes Scotland. "It's no wonder that their campaign is floundering," he adds.
But he's notably less fluent when asked why he and Gordon Brown keep missing each other on the campaign trail.
In recent weeks, a pattern has emerged of Darling being quite open to others at cross-party Better Together events, only for the more tribal Brown to trash the Coalition at a United with Labour gig days later.
The impression of Brown sabotaging Darling has fuelled speculation of a rift dating back to their fractious relationship as prime minister and chancellor.
Darling said the parallel events were "more accident than design", and voters should expect a Darling-Brown duet for the union at some point, although he doesn't say when.
"I'm quite sure that during the course of this campaign we will do these things because we take exactly the same view of this question, and we always have and we always will.
"I'm very clear that the Labour campaign is very complementary to what we're doing."
Despite Westminster rumours of him soon replacing Ed Balls as shadow chancellor, Darling insists he will lead Better Together all the way to the referendum, and not take on other roles.
"This is something that I didn't take on lightly, but it's something I'm totally committed to. It's the biggest single question in politics, not just in Scotland but in the whole of the UK. I'm not going anywhere else."