LH, I hate to ask this question, but I'm going to do so.

Did the US and its allies know, beyond a shadow of a doubt, that the situation in Japan was so bad that a blockade would bring about surrender? Unconditional surrender, as was demanded? Or is your thinking based on 20/20 hindsight?

Another point to be raised, and one which is definitely ethical and possibly moral, is whether or not it was more humane to incinerate several thousands and destroy two cities or literally starve thousand upon thousands to death by a blockade and continued air attacks. Remember that more people died in Tokyo and Dresden from firestorms than died in Hiroshima and Nagasaki combined -- and the cities were just as devastated. It goes back to the questions of the greatest good for the greatest number and whether, if there must be a war, it isn't better to end it as quickly as possible (e.g., Hiroshima and Nagasaki) or kill thousands and thousands (e.g. Dresden, Tokyo, the Western Front, and lots more) while bankrupting countries in more than financial ways.

I'm not going to try to answer what better minds than I have wrestled with.