“THE MOST ELEGANT of all colors” is how Christian Dior described black. He might have added: and the most powerful. That’s how the many dramatic black clothes on the spring runways came across—at Valentino, Narciso Rodriguez, the Row, Simone Rocha and Celine, to name just a few of the labels that went dark. It’s a cliché that black is a perennial in fashion, but until this spring’s concentration of expressive black, the color seemed to have receded, perhaps because it doesn’t read well on Instagram. That’s why this resurgence was so noticeable—and bracing. The small screen feeds instant gratification with flashiness...