"It's black over Bill's Mother's"

As a small child I would often hear my Mother using this expression on a day like today with torrential rain and low, heavy clouds. Who was Bill, who was his Mother, and where did they live? I never found out, but did find it odd at times when they appeared to have moved as the big cloud was seen  in a different direction!  Does anyone know the origin of this saying? The extra is a shot of the local river taken from the same spot as on the 9th Jan with the water level at least a couple of feet higher.