Initially from the title, I had assumed that the song would relate the sufferings of a newly-married woman due to the long absences of her husband and an imperious mother-in-law. However, looking at the lyrics by Sonosuke Sato（佐藤惣之助）, they themselves tell of the strong love between husband and wife, although the movie itself as seen above in the karaoke video has the main character indeed suffering when she is blinded by what seems to be a stray bullet.
"Niizuma Kagami" was a duet originally performed by Akiko Futaba（二葉あき子）and Noboru Kirishima（霧島昇）. According to her Wikipedia profile, the Hiroshima-born Futaba was once one of the most popular female singers in Japan and participated in the first 10 years of the Kohaku Utagassen making the transition from radio into television. Along with the duet performed on last Tuesday's show, I've also enjoyed this original version as an enka/kayo waltz which has that mix of melancholy and devotion.
Futaba debuted in 1936 and retired in 2003 at the age of around 88. She passed away in 2011 when she was 96.
Y'know...I look at that performance of the Kohaku in the year of my birth and I wonder what it must have been like to view the show back in those relatively early days.