The recent YouGov figures for Blair show a worse level of support than Corbyn. He'll never return to frontline politics in the commons, his name is too toxic.
Labour need to constantly remind voters it was the tories that put the EU referendum on the table, half the Tory party campaigned to leave and it will be the Tory Party that negotiates our exit. A Labour win in 2015 would have resulted in no EU referendum.
Whilst it's true now 52% voted out I think further down the line once it all unravels a fair chunk of that number will have regrets. In the short term it may be risky but a solid and sustained position in the long run will put them in a decent position to seriously question Tory economic credibility. A weak £ means import costs will rise significantly - Gas / Oil / Food etc, inflation is bound to creep up at a rate well beyond wage rises, people year on year will get poorer.
It's a risk and yes it questions the wisdom of the 52% and the democratic process but if they are proved to be right it's a massive opening. Given the polls they have nothing to lose, they need to gamble.