The French original of "Diabolique". It was filmed in very murky black and white; the ending could literally cause a heart attack.

BTW, someone mentioned "War of the Worlds" as being a black and white movie. The 1950s version with Gene Barry was in color and scary mostly because of how loud it was. I saw that movie at the Orpheum Theater in Los Angeles; it was weird sitting a couple of miles from City Hall watching it be 'destroyed'.