Speaking to the Progress centre-left think-tank in London, Mr Blair said there is a sense of optimism about Better Together and its arguments for keeping Scotland in the UK.
Better Together is led by Scottish Labour MP Alistair Darling, who held a string of cabinet posts under Mr Blair before rising to chancellor under Gordon Brown.
Mr Blair has kept a low-profile in the Scottish independence campaign with few public pronouncements on the issue.
His latest comments have been described as "a huge embarrassment for the No camp" by the SNP.
When asked if Better Together and the Labour leadership done enough to save the union, Mr Blair said: "I hope so, I believe so.
"I think the arguments of the Better Together campaign have got stronger as time has gone on. We will see."
He said that "there is a sense of optimism" about Better Together.
SNP Westminster leader Angus Robertson said: "Getting the support of Tony Blair on the day it is reported he is to receive a warning letter from the Chilcot Inquiry as the architect of the illegal war in Iraq can only be a huge embarrassment for the No camp.
"Mr Blair has been kept well away from the referendum debate by the No campaign for obvious reasons, until now.
"His illegal war in Iraq was a calamity, and an avoidable one at that, and the people of Scotland will not forget it."
He said: "Only with a Yes vote in September can Scotland avoid any future illegal Westminster wars and decide a foreign policy based on Scotland's interests."