Back in the late 50s and early 60s, Hollywood was trying desperately to compete with TV. That's why so many big epics came out at that time. They wanted going to the movies to be "an EVENT". Many of those epics began with what was called an "overture", which consisted of either a black screen or a painting representative of the movie accompanied by a medley of themes from the soundtrack.
They would also have an intermission in the middle with music playing so you could go buy snacks and go pee etc. It was just a kind of pretentious way to make the movie seem more important.
They would play the overture before the curtains opened. Since TVs don't have curtains, this doesn't really translate. I was a bit confused about this when I first saw 2001: A Space Odyssey.
There was no black screen at the theater. The screen just didn't have anything projected on it
The soundtrack was on the film itself so when the overture music began, the projector was running. In other words, something was being projected, even it it was just blacked out frames.
The general practice was, the projector started, the house lights dimmed and the curtain remained closed. But the blacked out frames were still barely discernible on the curtain. When the title and opening credits began, the curtain opened.
But as previously mentioned, sometimes a static image appeared when the overture was playing. Obviously, in this case the house lights were all the way down and the curtain was open. How the West was Won is a good example.
I wasn’t sure if it was an overture or the projectionist had technical problems because the same theater a few weeks earlier had some similar technical issues during the preview trailers. I wish they would have had “Overture” written on the screen.