One of the things that separates engineering from theoretical science is the never-ending presence of design criteria that are not directly related to the problem at hand. I once had a small job (supporting new ventilation equipment in a hospital) balloon into a medium-sized one because the best (using engineering criteria) design choice of installing the supports below was not possible, because the space below was a patient ward that could not be disturbed. The engineering design was heavily influenced by a non-engineering issue, and one that was contingent on the exact location of the work within the building. Had the equipment (which was not related to that ward) been above office space, the easier design could have been used.
The picture above is one I’ve been trying to get for years, but the phenomenon it shows is just about gone. During World War II, cities on the Atlantic and Pacific coasts were supposed to reduce their night-time lighting. It was rarely a full blackout, but a lot of effort was put into reducing the amount of sky glow created by electric lights. It was sold as reducing the likelihood that US cities would be subject to aerial bombing, but it had the more immediate effect of making it more difficult for submarines to find ships by reducing back-lighting.
A lot of old loft buildings in New York had skylights, as did most elevator shafts. The skylights at the roofs of elevator bulkheads were specifically to allow for rapid venting of the shaft during a fire: a few swings of an axe and voilà! the shaft was vented. In any case, since I first started building investigation, I’ve seen skylights covered or partially covered with black paint. No one seemed to know why or when that happened, and the painted skylights have gradually disappeared, the victims of modernization and repair work. In “Blackout Lighting,” reprinted in the Journal of the American Society for Naval Engineers in 1943 from a December 1942 article in the General Electric Review, an electrical engineering journal published by GE, blacking out skylights was specifically recommended as part fo the war effort.
This is not proof that black paint dates from the 1940s. But it’s a better explanation than any other I’ve come across for this. If it were tar or some other coating that might have provided waterproofing, I could believe it was slopped across the glass by accident when the joints were being sealed, but it’s specifically paint. The disappearance of these painted skylights between the late 1980s and today suggests that they were all painted at one time and not again since.