I finally saw THE black dress in person.

The museum label read:

Henri Matisse
French, 1869-1954

Femme au chapeau (Woman with a Hat)
1905
Oil on canvas

A defining modernist painting, Woman with a Hat ignited controversy when it was first shown in Paris at the 1905 Salon d'Automne, due in part to Matisse's use of the unexpected, high-keyed, and shocking colors to depict his wife, Amélie. When asked what color dress she was wearing when the portrait was made, the artist is said to have replied, "Black, of course." The hat—most likely Amélie's own creation—prompted a San Francisco art critic to remark in 1928 that the work should be retitled "Landscape, for it is a very jungle of a hat, with fruits, flowers and birds."