Stalin knew the end of the fight against Germany would free up his vast armed forces to invade Japanese-held territory in Asia, such as resource-rich Manchuria. The USSR declared war after the Hiroshima bombing but before the Nagasaki bombing. It's intention was clearly to grab territory before Japan surrendered
Some think it was called this because of how the skin turned dark at the late stages of the disease, but it was more likely called "Black" to reflect the dark and horrible time in history.
