I have never seen the B/W episodes and am just beginning to see the color episodes. Now I watch them daily on Odyssey. My question is...How did the first Mrs. Douglas die and when? Was this explained during the B/W episodes?

No Susie it was never fully explained. In the very first episode, the oldest brother Mike (who married and was written out of the show when it went to color in 1965) was 18,Middle brother Robbie was 14, and youngest son Chip was 8 years old. There is a Season 2 episode where Chip writes a composition on "What my mother means to me" and there is a scene where he is looking at a photograph of the mother (which I think looks very much like Fred MacMurray's real-life wife, June Haver, and asks the oldest brother Mike about what she was like. He ends up writing about the grandfather, Bub (played by I Love Lucy's William Frawley.)
Bub left the show before the last Black and White Season finished and was replaced by William Demarest as his searfaring brother,
Charley O'Casey.