From what I hear the black cameras were black so as to minimise any reflections from the flash bouncing off the camera & reflecting in the subjects eyes. Also black was less obtrusive in use as it didn't shine as much. Ironically the black bodiesmade of brass are usually not as strong as the chromed bodies (they dent more easily). I believe that black cameras took off because Cartier Bresson used to use black tape on his Leica's.