Well, here's how it works
He would no longer be PM. The Labour Party would select a new leader, and that person would become PM. The likelihood of the Conservative Party making him leader would approach 0; it's extremely unlikely that they would take the person who was disloyal to his party and make them the leader of their party... such a move smacks of political opportunism, and in parliamentary democracies, the person who would do that is not to be trusted in a party. Then, come election time (and him voluntarily leaving the party would NOT be a pretext for calling an election) it would be the luck of the vote as to whether the person who is leading the Conservative Party would become PM... who would almost certainly NOT be Tony Blair.
For him to do that would be comitting political suicide.
