Hell, I’ve been looking for a post WWII black/white film about London being deserted because of an old man with a bomb. I saw it a couple times as a child, but never saw it again. It showed empty streets, etc.
From your description, I think that’s “Seven Days to Noon”
http://www.imdb.com/title/tt0042949/
Last Man on Earth was black and white but that was Vincent Price in Italy.
“Seven Days to Noon” (1950) is the name of that film.