Could be The World, The Flesh, and The Devil, with Harry Belafonte. He’s a maintanence supervisor for the sewer system in New York when the balloon goes up. Being underground saved him, but disease or radiation or whatever killed off nearly everybody else before he can get to the surface again. Finds another person, a white woman, and that makes the relationship iffy for those days. Then the two find a third, a white man. After a lot of tension things settle down and the last scene is of the three of them walking away arm in arm, with the words “The Beginning” on the screen. I only saw it once, a long time ago on TV. so my details may be off, but check if that’s the one.