Hi,
Lovely dress with great style. Definitely not 1920s. It has a mid to late 1930s style to it. I think the large bold pattern is fine for later 1930s, it has a sort of Deco tribal exotica appeal to it. The key is the material's fiber content. In your photos, the gold threads look more like Lurex (modern) than lame', to me. Snip off some of the gold threads from the seam allowance and see what happens. If it melts, its likely Lurex or a similar modern synthetic metallic coating. Do a burn test on the black material also. Those will be your best clues.
If it is a 1970s or other era dress made in the 1930s style, and is a synthetic fabric, I would think it would be very difficult to dye it black from another color. Synthetics do not, asa rule, take dyes as well as natural fibers, and with black it can be very hard to get a true color.....you often wind up with purple shading. Or perhaps it was already black or gray and was re-dyed to cover imperfections or fading.
Since we cannot see it in person, maybe you can show us a close up of the gold threads.