My home was built in 1950, I have a basement kitchen and two kitchen sinks share the same standpipe. About 3 years ago the floor drain was backing up, I called a plumber and he augered out the floor drain and opened the standpipe and cleared out a bunch of nasty grease and stuff. Ever since that experience I have kept grease and food from going down the drain, there is no dishwasher or garbage disposal. Now, when I run water upstairs in the kitchen, the basement sink backs up with black grit. I poured boiling water down the upstairs sink and could feel the heat along the standpipe until just above the basement sink. The grit is black and not greasy or rusty looking, very much like sand and pebbles.