WHEN MICHELLE DUNCAN showed her first collection at New York fashion week in February, she didn’t anticipate being in the vanguard of one of the season’s dark-horse trends. A beauty-industry executive by day, she drew on her own personal style for the line, creating an aesthetic she christened “goth girl gone corporate.” The collection featured fitted, heavy wool dresses in sober colors, a half-pleated tartan skirt and a sooty, high-collared coat with an ankle-skimming hemline trimmed in peacock feathers that would befit...