If a top player can lose his concetration by something as trivial as a feather, he's got some issues in the mental department.
it was between first and second serve at 2*-3 in the tie-break, honestly I've seen Djokovic lose his concentration for similar reasons in the past
there was a lot of time between the first and the second serve, and Djokovic was clearly prepared to attack on second serve, I don't think Murray completely lost his focus but only he lacked the rhythm that you usually have after hitting a first serve.
He hit his second serve wide (not hugely wide, I think).
And actually Murray's first serve was very nearly in.
Besides, it seems that he called the trainer for his blister precisely at the sides change, the blister may have been "opened" precisely at that moment.