The mention of the fate that awaited the POWs is timely.
During the early 60's I met an ex POW and he was in no doubt that the 'Bomb' saved him and many others.
One important point that should not be overlooked is the political dimension. Imagine Harry Truman giving one of his fireside chats. ' My fellow Americans, we have a weapon that will end the war against Japan, but it is so terrible I have decided not to use it '. How long would it be before the white House switchboard melted with the weight of incoming calls or for impeachment proceedings to begin.
The blockade was an option but the Japanese military government would still have fought on ( remember they killed the General commanding the Imperial Guard and attempted to kidnap the emperor to stop the surrender broadcast ATER the second bomb) and the civilian casualties from starvation and disease would have been in their millions. The USSR would almost certainly have invaded the Japanese mainland, giving rise to claims that they alone were attempting to take the land fight to the Japanese homeland. Many more Japanese would have died AS A RESULT. IMHO President Truman did the only thing in the circumstances. Do you know of an instance in all history where a combatant has not used a superior weapon that he had developed ?.
Finally the 'peaceful' occupation of the country the retention of the Emperor and the benign 'rule' of the 'American Cesar ' set the scene for the rapid economic and political recovery of Japan in the post war era.