If you want to find the origins of a uniquely American style of dress for women, you can start with the shirtwaist blouse and dark skirt so popular at the beginning of the twentieth century. Although it was worn in other countries, it was most beloved in the United States. Europeans often complained that American women travelers, clad in their Gibson Girl outfits, all looked the same.
This photo, taken from a book by African American leader Thomas O. Fuller, shows that the shirtwaist and skirt combination had interracial and inter-generational appeal. The combination could be very versatile. Several of the older women wear a dark blouse with their dark skirts, making the outfit look more like a dress. I particularly like the striped blouse worn by an older woman on the right in the front. The white collar near her face, combined with the shiny buckle on her skirt, gives the outfit a very dressy appearance.
Given the similarities, you might think there was a dress code for this Bible training class. I doubt that was the case. If the photos I have seen are at all representative, this is how most American women dressed in 1910.