Robert Mnuchin, the Wall Street pioneer who became one of New York’s most respected art dealers and a fixture at blue-chip auctions, died on Friday at his home in Bridgewater, Connecticut. He was 92. His death, first reported by the New York Times, was confirmed by his stepdaughter, Lisa Hedley Wick.  Mnuchin was unusual in having two long and highly successful careers. At Goldman Sachs, he was a central figure in the rise of block trading in the 1960s and ’70s, helping build the firm’s institutional equities business under managing partner Gustave Levy. By 1978, the Wall Street Journal described him as “the acknowledged dean of block traders,” rivaled only by Salomon Brothers’ Michael Bloomberg. His skill on the trading floor—an encyclopedic recall of buyers and sellers, an instinct for pressure and timing—made him a legend inside the firm. He became a partner in 1967, co-headed trading and arbitrage by 1976, and joined the firm’s powerful management committee in 1980. He retired in 1990.  That same set of instincts—competitive, analytic, and deeply interpersonal—animated his second act. After …