But the former chancellor insisted he was "increasingly confident" that Scots would vote against leaving the UK when they go to the polls in just over three weeks' time.
He spoke out after last night's second and final head-to-head television clash with Scottish First Minister Alex Salmond - with a snap poll after the BBC debate suggesting the SNP leader came out on top.
Research by ICM for the Guardian newspaper showed 71% of people questioned thought Mr Salmond had been the better performer in the programme, compared with 29% for Mr Darling.
A similar survey after the first debate earlier this month suggested the former chancellor had scored a narrow victory over the SNP leader.
In both televised debates, the issue of which currency an independent Scotland would use was at the heart of the clashes between the rival politicians
Mr Darling again repeatedly pressed the First Minister to set out his plan B for a currency if a formal deal could not be agreed with the rest of the UK to allow an independent Scotland to retain the pound.
But the former chancellor appeared to accept last night that Scotland could use the pound regardless of whether Westminster signed up to such an arrangement.
He said: ''Of course we can use the pound... we could use the rouble, we could use the dollar, we could use the yen. We could use anything we want.''
Today, as he visited a factory in Inchinnan, Renfrewshire, Mr Darling said the No campaign had been "right to focus on currency", adding: "One of the things that is striking in the last three weeks is the amount of people who have come up to me and said 'What about this plan B?' is quite remarkable."
When asked if changes would be made to the pro-UK campaign following last night's debate, he insisted: "Absolutely not."
Postal votes for the September 18 ballot are being sent out from today, and Mr Darling added that as the "finishing post" comes into sight, Scots are increasingly focused on the decision to be made.
He said: "There's a very clear choice here and I think people can see that, people are very focused now, especially as they can see the finishing post.
"I'm increasingly confident we will win, but it is our job to win well and to win well we've got to carry on campaigning up untill 10pm on September 18.
"For the last two years, business after business has told me that they are afraid to say things for fear of people shouting out at them... there's the cybernat activity on the internet.
"But in the next couple of weeks or so, I think as people can see that polling day is coming up, businesses in particular are so conscious of the fact that this decision is so big and could have major repercussions on what they do, I think you will find them speaking out a lot more."
The Better Together leader continued: "If you look at all the evidence, all the polls that have been published for the last few months, they consistently show us with a lead, most of them a double-digit lead.
"I'm not complacent - a lot can happen in the next three weeks - but we have momentum. We're making good progress, our activity on the ground, on the streets, has been stepped up dramatically, the results we're getting in are extremely encouraging. I'm very optimistic but I'm not complacent.
"I've always said I thought this would be a fight right down to the wire, but I am increasingly confident."
With people starting to vote by post from this week, Mr Darling claimed the nationalists had failed to answer key questions about independence.
Speaking on a visit to Peak Scientific UK's site in Inchinnan, the Labour MP said: "The postal votes are going out today, people will start to vote, but for the many undecided voters they still don't have clear answers on some basic questions, like what currency will we use, who will pay for our pensions, who will pay for public services like the NHS.
"I think a growing number are realising there is a better choice, we can have the best of both worlds with a strong Scottish Parliament with more powers guaranteed, but at the same time we've got the strength and security of the United Kingdom."
He continued: "This company is an example of one which has benefited from the fact that we have the UK, particularly in relation to their sales throughout the world. This is a company that sells throughout the world, we have over 270 embassies throughout the world ready to promote trade and provide assistance.
"But also on a practical level a firm like this needs to know what currency it is going to be dealing with, that's critically important for its development."
He also said that while television debates were a key part of the democratic process, the "big debate that actually matters is in people's sitting rooms and people's kitchens, that's where the decision is going to be made".
Mr Darling said: "If you look at these television debates, they add to the debate, in many ways people will look at that. But I think people's minds are being made up by what they've been reading in newspapers, what they've been seeing on television for the last two and a half years.
"I've always taken the view these debates are part of the democratic process... but the decision is actually going to be made in people's houses rather than on the television sets.
"These debates are part of a process but there is a far bigger debate going on. It's not taking place on the the television channels, it's taking place in people's houses.
"At the start of this campaign I said the economic decisions, the big decisions over currency, how we pay for public services, our dependence on a very volatile North Sea oil would be key.
"At the end of the day undecided voters, what is worrying them, is the risks and uncertainties that come with independence, that is what is worrying them far more than anything else.
"After over three hours of prime time television debate the fundamental questions in front of people remain unanswered."
He stressed more powers would come to Scotland in the event of a No vote, but Labour, the Conservatives and the Liberal Democrats have not reached an agreement over what these would definitely be, with the three parties having put forward separate proposals.
Despite that, Mr Darling said there was "far more agreement now than there ever was in the past" on the issue of further devolution.
He said: "Frankly, I'd rather be inside the United Kingdom arguing about the fine detail about further powers than outside the United Kingdom begging to be let into a currency union."
He continued: "Most people in Scotland do want more powers for the Scottish Parliament, but most people can see the benefits that come from being part of something bigger, the jobs that comes from that as well as the security that comes."
Mr Salmond told Sky News he had "argued persuasively" on currency, setting out his position that it was "common sense for a common currency".
He argued that the Yes campaign was winning "decisively" on the key issues of protecting public services and creating jobs in Scotland.
The First Minister said: "He (Alistair Darling) had his chance and he muffed it last night. The currency bluff has been called. The Scottish people are calling that bluff.
"The No campaign had their chance, their bluff has been called. People have, I think, overwhelmingly in Scotland now in poll after poll shown that we want to keep the pound. That is the decision that the Scottish people are being asked to make.
"That is the sovereign will of the Scottish people. I have explained in enormous detail why it's good sense for Scotland, it's good sense for the rest of the UK. I've explained what the other currency options are for Scotland. But we're fighting a campaign to get a mandate from the Scottish people on common sense for a common currency.
"That's what won the debate last night and that's the message that's going to resonate over the next three weeks.
"Have a look at what the people thought about it, the audience thought about it. This debate is moving on, with people realising that to protect our NHS we have to have financial control, to have job-creating powers for Scotland, that's what won the debate."
Mr Salmond accused Mr Darling of being the "front man for the Conservative Party" in the campaign.
He said: "Alistair Darling is in alliance with the Conservative Party in this campaign. That is what the No campaign is.
"Let's have the real leader of the No campaign, David Cameron. Let's have him in Scotland now, let's see if he can do any better that Alistair Darling did. I don't think he will.
"Let's see if Mr Cameron is prepared to come to Scotland and have the debate."
Downing Street said David Cameron - who is on holiday in Cornwall - had not watched last night's debate in full.
"The Prime Minister watched the news reports of the debate," a spokeswoman said.
Mr Cameron also only saw "highlights" of the first debate, she added.
However, the spokeswoman denied that meant the premier had not engaged with a key part of the campaign.
"The key moment is when people go to vote on September 18," she added.