Actually, the first version of the question asked Blair who he wanted as the next leader of the Labour Party. Thats when he was stopped. He later rephrased it as the next PM.
http://news.bbc.co.uk/1/hi/uk_politics/6106362.stmThe question was phrased as follows
'David Cameron: I know [Tony Blair] doesn't want to talk about the chancellor - he can't even mention his name - but let's just spend a moment on it. Let me put the question I put to him three weeks ago. Back in January, the prime minister said: 'I'm absolutely happy that Gordon Brown will be my successor.' Now does the prime minister... (interrupted)
Michael Martin: Or-order. Or-order. I have let the Rt Hon Gentleman away with that before, but this matter is not going to be belaboured, because the prime minister is here to talk about business of the government.'
Cameron did not directly mention Labour or the Labour leadership. Infact it was the Speaker himself who referred to the Labour leadership.