The phrase "black gown" typically refers to a long, black robe worn by certain professionals, such as judges or academics, to denote their authority and expertise in their field. Full definition
I asked her to dress the girls in black gowns and fur cover - ups. (brides.com)
On the other end of the spectrum, some stars impressed in sophisticated black gowns with beaded accents and contrasting details. (thefashionspot.com.au)
In addition to white there was also a token number of black gowns as well. (myredglasses.com)