I disagree. Japanese culture was (and probably still is) exceptionally different from any other mentality at the time, still worshiping a supreme monarch and being extremely, extremely nationalistic in nature. This may be difficult to comprehend if you come from a country which has lived for a long period of time as democratic, but there's no real doubt that the Japanese wouldn't have surrendered.
This can be proven in many ways. The most obvious example would be the kamikaze pilots, followed by fanatical suicide fighters on the battles of Iwo Jima and other island territories which held no real value other than just being something under the Japanese flag.
Furthermore, you can find a lot of post-war literature published by many of the Japanese military minds and other such people in positions of power who advocated fighting to the very last man, woman, and child. Being a country with no real form of freedom of speech, thought, or creativity, it was pretty easy to manipulate the populace.
Granted, I think they would have surrendered even without a nuclear bomb, but something had to jolt them out of their nationalistic stupor. If the firebombings of Tokyo didn't work (and it didn't... but rather fueled their spirit just like the blitzkrieg raids on London), it wasn't likely that a naval blockade or even a full scale invasion would have.