The 16-year-old Hurd edged Black by 0.101 points, finishing with 55.232 points in her world championships debut. Russia's Eremina Elena was third at 54.799.
"It's crazy that I was even competing here," said Hurd, from Middletown, Delaware. "To think that I won is just the most surreal feeling in the world."
American Ragan Smith, the favorite to win the gold medal, was forced to withdraw from the final after injuring her ankle during the vault warm up at Olympic Stadium.
Black is a five-time Canadian champion.
"I'm standing here with the silver medal, so I'm pretty happy with that," said Black, from Halifax, Nova Scotia. "There's ups and downs in gymnastics, there are mistakes that are made. It's part of the sport. There's probably a bunch of athletes who would love to be in my position right now. I just feel so fortunate to have had this opportunity and to be here tonight."
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