“The rise of African-American artists is part of a broader tendency to re-evaluate neglected artists that’s been going on for a few years,” said Candace Worth, an art adviser based in New York. “Art history isn’t just about the big Ab-Ex guys any more,” she added, referring to postwar painters such as Pollock, Rothko, de Kooning and Richter, who have long dominated the auction scene. “We’re opening a conversation, and the market is playing catch-up.”