The analogy of sleepwalking is one much loved by politicians to describe a group of people making a decision without really being aware of what they’re doing or what the consequences might be. And in most cases, including this one, it’s deeply insulting to those making the decision – usually the electorate.
Blair is in effect saying that the members of the Labour Party could not possibly arrive at the decision which looks increasingly likely as a result of a conscious thought process after weighing up the options, so they must be arriving at their decision without thinking too deeply about it. It’s not far short of saying that the members of his party are too stupid to be trusted to take the right decision.
In fairness, though, looking at some of the leadership choices the Labour Party has made in the recent past, perhaps he has, unintentionally, made a valid point.