To me, this is about a love affair with heroin. (I was never the same on the black sand.)
the lines, "Who's going to love yo buried underground" and "who's going to mind, when the end is nigh?" refer to the danger of it, but really, "Who's going to love him?"
I'm not sure how much sense I'm making, but that's what I'm getting out of it.

I believe that this song is all about falling in love with someone in the midst of a war. The "black sand" is meant to be a metaphor for the massive amounts of ashes covering the world. Also, the song is about embracing that love, even in the face of disaster, because as Jenny's lover inquires, "Who's gonna love you, buried underground?", i.e., love and be loved before you die.

This may be wrong, but, I was also thinking that the mother's "mate" in the song is some sort of prisoner of war, from whatever side is in opposition to hers. Thus, they must love each other in the time they have before "all that's lost is found".

Beautiful song, though. The ending is really powerful.

I saw Jenny Lewis in concert and she said about this song, "I think this song is about the afterlife, but I'm not sure yet." Makes sense when you listen to Jenny's music. She has a great ability to describe an image, a scene, and convey some metaphorical meaning in it. I can believe the image of black sand, maybe wet sand on the beach at night with just slight moonlight illuminating, giving off the appearance of black sand, would lead someone to think of death and the afterlife.

On the south coast of france there are beaches of sable noir, and I always thought of those from this song. That's pretty interesting though, what you guys said- it probably is a heroin afterlife love affair.